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An Introduction to the Zen of Python | KEY
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An Introduction to the Zen of Python

The document discusses key principles of Python style and design according to the Zen of Python. It emphasizes that Python code should be beautiful, explicit, and simple rather than complex. It provides examples of simple expressions, boolean values, for loops, and defining a class in Python to illustrate these principles. The class definition example shows how to define a class with documentation, initialization, and methods that reference object attributes using self.

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An Introduction to the Zen of Python Doug Hellmann February, 2011
The Zen of Python by Tim Peters • Beautiful is better than ugly. • There should be one – and • Explicit is better than implicit. preferably only one – obvious way • Simple is better than complex. to do it. • Complex is better than • Although that way may not be complicated. obvious at first unless you're • Flat is better than nested. Dutch. • Sparse is better than dense. • Now is better than never. • Readability counts. • Although never is often better than • Special cases aren't special *right* now. enough to break the rules. • If the implementation is hard to • Although practicality beats purity. explain, it's a bad idea. • Errors should never pass silently. • If the implementation is easy to • Unless explicitly silenced. explain, it may be a good idea. • In the face of ambiguity, refuse the • Namespaces are one honking temptation to guess. great idea – let's do more of those!
Beautiful is better than ugly. • Write programs for human readers • Simple expression syntax • Consistent syntax and behavior
Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
Explicit is better than implicit. • Boolean type • No “hidden” loop variables • Use “self” to refer to object inside a method
Boolean Values 1 t = True 2 f = False 3 4 if t: 5 print 't is true' $ python booleans.py 6 t is true 7 if not f: f is false 8 print 'f is false' True 9 False 10 a = t or f 11 b = t and f 12 13 print a 14 print b
for loops $ python for_loop.py 1 print 'Numbers:' Numbers: 2 0 3 for i in range(5): 1 4 print i 2 5 3 6 print 4 7 print 'Letters:' 8 Letters: 9 for c in [ 'a', 'b', 'c' ]: a 10 print c b c
Defining a Class 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
Defining a Class 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
Defining a Class 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
Defining a Class 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
Defining a Class 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
Simple is better than complex. • Garbage collection • print statement • Interactive interpreter for experimentation
Printing 1 # -*- encoding: utf-8 -*- 2 3 print 'string:', 'string' 4 5 print 'unicode:', u'üñîçø∂é string' 6 7 print 'number:', 5 * 5 8 9 my_list = [ 1, 'a', None ] 10 print 'list:', my_list 11 12 my_dict = { 'a':'a value', 'b':'b value' } 13 print 'dict:', my_dict $ python printing.py string: string unicode: üñîçø∂é string number: 25 list: [1, 'a', None] dict: {'a': 'a value', 'b': 'b value'}
Printing, with printf-style Formatting 1 # -*- encoding: utf-8 -*- 2 3 my_list = [ 1, 'a', None ] 4 my_dict = { 'a':'a value', 'b':'b value' } 5 6 args = ('string', u'üñîçø∂é string', 5 * 5, 7 my_list, my_dict) 8 9 print ''' 10 string : %s 11 unicode: %s 12 number : %d $ python printing_with_format.py 13 list : %s 14 dict : %s string : string 15 ''' % args unicode: üñîçø∂é string number : 25 list : [1, 'a', None] dict : {'a': 'a value', 'b': 'b value'}
Interactive Interpreter
Complex is better than complicated. • Do complex things in a “Pythonic” way • SciPy and NumPy for advanced science and math
Interactive Graphic SciPy, NumPy, matplotlib Environment
NumPy Performance Standard Python NumPy 1 a = range(10000000) 1 import numpy as np 2 b = range(10000000) 2 a = np.arange(10000000) 3 c = [] 3 b = np.arange(10000000) 4 for i in range(len(a)): 4 c = a + b 5 c.append(a[i] + b[i]) 5-10 seconds “nearly instantaneous”
Flat is better than nested. • Wide variety of modules in standard library • Namespace is flat • No need to use long names: com.company.java.blah.blah
Standard Library Modules: Networking • socket • asyncore • select • asynchat • SocketServer • xmlrpclib • BaseHTTPServer • SimpleXMLRPCServer
Standard Library Modules: Internet Protocols • urllib • poplib • httplib • imaplib • ftplib • json • smtpd • smtplib
Standard Library Modules: Command Line • getopt • fileinput • optparse • cmd • argparse • readline
Standard Library Modules: Parallel Processing • subprocess • multiprocessing • threading • Queue
Standard Library Modules: Databases • anydbm • shelve • pickle • sqlite3
Standard Library Modules: Miscellaneous • datetime • struct • grp • ctypes • pwd • xml • profile • tarfile • timeit • csv
Sparse is better than dense. • Standard library stays “shallow” • http://pypi.python.org/ • Allows separate release schedules • pip (“pip installs Python packages”)
Readability counts. • Whitespace for block structures • Minimal punctuation • Built-in documentation • Extensive online documentation at http:// docs.python.org/
Code Blocks 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
Code Blocks 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
Minimal Punctuation 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
Minimal Punctuation 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
Inline Documentation 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
Inline Documentation 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
Special cases aren’t special enough to break the rules. • Everything is an object • Methods and functions differ by scope • Dynamic typing and runtime attribute lookup • Most “features” in external modules
String Methods $ python string_methods.py 1 print ' string value '.split() ['string', 'value'] 2 print ' string value '.strip() string value 3 print 'str' in ' string value ' True 4 print 'CHUGALUG'.startswith('C') True 5 print 'CHUGALUG'.endswith('end') False 6 print ':'.join(['a', 'b', 'c']) a:b:c
String Methods $ python string_methods.py 1 print ' string value '.split() ['string', 'value'] 2 print ' string value '.strip() string value 3 print 'str' in ' string value ' True 4 print 'CHUGALUG'.startswith('C') True 5 print 'CHUGALUG'.endswith('end') False 6 print ':'.join(['a', 'b', 'c']) a:b:c
String Methods $ python string_methods.py 1 print ' string value '.split() ['string', 'value'] 2 print ' string value '.strip() string value 3 print 'str' in ' string value ' True 4 print 'CHUGALUG'.startswith('C') True 5 print 'CHUGALUG'.endswith('end') False 6 print ':'.join(['a', 'b', 'c']) a:b:c
String Methods $ python string_methods.py 1 print ' string value '.split() ['string', 'value'] 2 print ' string value '.strip() string value 3 print 'str' in ' string value ' True 4 print 'CHUGALUG'.startswith('C') True 5 print 'CHUGALUG'.endswith('end') False 6 print ':'.join(['a', 'b', 'c']) a:b:c
String Methods $ python string_methods.py 1 print ' string value '.split() ['string', 'value'] 2 print ' string value '.strip() string value 3 print 'str' in ' string value ' True 4 print 'CHUGALUG'.startswith('C') True 5 print 'CHUGALUG'.endswith('end') False 6 print ':'.join(['a', 'b', 'c']) a:b:c
String Methods $ python string_methods.py 1 print ' string value '.split() ['string', 'value'] 2 print ' string value '.strip() string value 3 print 'str' in ' string value ' True 4 print 'CHUGALUG'.startswith('C') True 5 print 'CHUGALUG'.endswith('end') False 6 print ':'.join(['a', 'b', 'c']) a:b:c
String Methods $ python string_methods.py 1 print ' string value '.split() ['string', 'value'] 2 print ' string value '.strip() string value 3 print 'str' in ' string value ' True 4 print 'CHUGALUG'.startswith('C') True 5 print 'CHUGALUG'.endswith('end') False 6 print ':'.join(['a', 'b', 'c']) a:b:c
Function and Method Scope 1 class MyClass(object): 2 3 def do_something(self, arg1, arg2): 4 return arg1 * arg2 5 6 def my_function(arg1, arg2): 7 return arg1 * arg2
Dynamic Attribute Lookup 1 class MyClass(object): 2 3 CLASS_ATTR = 'class value' 4 5 def __init__(self, arg): 6 self.attr = arg 7 8 o = MyClass('instance value') 9 print o.attr 10 print o.CLASS_ATTR $ python dynamic_attribute_lookup.py instance value class value
Dynamic Typing 1 class A(object): 2 def do_something(self): 3 print 'in A' 4 5 class B(object): $ python dynamic_typing.py 6 def do_something(self): in A 7 print 'in B' in B 8 9 def work_on_obj(c): 10 c.do_something() 11 12 a = A() 13 b = B() 14 15 work_on_obj(a) 16 work_on_obj(b)
Regular Expressions 1 import re 2 3 pattern = 'this' 4 text = 'Does this text match the pattern?' 5 6 match = re.search(pattern, text) 7 8 s = match.start() 9 e = match.end() 10 11 print 'Found :', match.re.pattern 12 print 'In :', match.string 13 print 'Range :', s, '-', e 14 print 'Substring:', text[s:e] $ python using_regex.py Found : this In : Does this text match the pattern? Range : 5 - 9 Substring: this
Although practicality beats purity. • Multiple Programming Models • OOP • Procedural • Functional
Procedural 1 def my_function(arg1, arg2): 2 """This function multiplies arg1 by arg2. 3 """ 4 return arg1 * arg2 5 6 results = [] 7 for i in range(5): 8 results.append(my_function(i, i)) 9 10 print results $ python procedural.py [0, 1, 4, 9, 16]
Functional: List Comprehension 1 results = [ i*i for i in range(5) ] 2 3 print results $ python functional.py [0, 1, 4, 9, 16]
Functional: List Comprehension 1 results = [ i*i for i in range(5) ] 2 3 print results $ python functional.py [0, 1, 4, 9, 16]
Functional: List Comprehension 1 results = [ i*i for i in range(5) ] 2 3 print results $ python functional.py [0, 1, 4, 9, 16]
Functional: List Comprehension 1 results = [ i*i for i in range(5) ] 2 3 print results $ python functional.py [0, 1, 4, 9, 16]
Functional: Generator Expression 1 results = ( i*i for i in range(5) ) 2 3 print results 4 5 for i in results: 6 print i 1 $ python generator_expr.py 2 <generator object <genexpr> at 0x1004d20a0> 3 0 4 1 5 4 6 9 7 16
Functional: Generators 1 def gen_values(n): 2 for i in range(n): 3 yield i*i 4 5 print gen_values(5) 6 7 for i in gen_values(5): 8 print i 1 $ python generator.py 2 <generator object gen_values at 0x1004d2050> 3 0 4 1 5 4 6 9 7 16
Errors should never pass silently. • Exception-based error handling • Tracebacks aid debugging
Exceptions and Tracebacks 1 def f(n): 2 if n > 0: 3 return n * f(n-1) 4 return 1 5 6 print f('5') $ python traceback.py Traceback (most recent call last): File ".../traceback.py", line 6, in <module> print f('5') File ".../traceback.py", line 3, in f return n * f(n-1) TypeError: unsupported operand type(s) for -: 'str' and 'int'
Raising Exceptions 1 def f(n): 2 if not isinstance(n, int): 3 raise TypeError('n should be an int') 4 if n > 0: 5 return n * f(n-1) 6 return 1 7 8 print f('5') $ python raising.py Traceback (most recent call last): File ".../raising.py", line 8, in <module> print f('5') File ".../raising.py", line 3, in f raise TypeError('n should be an int') TypeError: n should be an int
Raising Exceptions 1 def f(n): 2 if not isinstance(n, int): 3 raise TypeError('n should be an int') 4 if n > 0: 5 return n * f(n-1) 6 return 1 7 8 print f('5') $ python raising.py Traceback (most recent call last): File ".../raising.py", line 8, in <module> print f('5') File ".../raising.py", line 3, in f raise TypeError('n should be an int') TypeError: n should be an int
Unless explicitly silenced. • Catch exceptions with try:except • Process • Convert • Ignore
Catching Exceptions 1 def f(n): 2 if not isinstance(n, int): 3 raise TypeError('n should be an int') 4 if n > 0: 5 return n * f(n-1) 6 return 1 7 8 try: 9 print f('5') 10 except TypeError, err: 11 print 'Had an error:', err 12 else: 13 print 'No error' 14 finally: 15 print 'Always run' $ python catching.py Had an error: n should be an int Always run
In the face of ambiguity, refuse the temptation to guess. • Coerce types only when not surprising • Cannot add strings and numbers • Can multiply them!
Type Coercion 1 print 5 * 1.0 2 print 2 ** 100 3 print 2 * 'abc' 4 print 2 + 'abc' 1 $ python type_coercion.py 2 5.0 3 1267650600228229401496703205376 4 abcabc 5 Traceback (most recent call last): 6 File ".../type_coercion.py", line 4, in <module> 7 print 2 + 'abc' 8 TypeError: unsupported operand type(s) for +: 'int' and 'str'
Type Coercion 1 print 5 * 1.0 2 print 2 ** 100 3 print 2 * 'abc' 4 print 2 + 'abc' 1 $ python type_coercion.py 2 5.0 3 1267650600228229401496703205376 4 abcabc 5 Traceback (most recent call last): 6 File ".../type_coercion.py", line 4, in <module> 7 print 2 + 'abc' 8 TypeError: unsupported operand type(s) for +: 'int' and 'str'
Type Coercion 1 print 5 * 1.0 2 print 2 ** 100 3 print 2 * 'abc' 4 print 2 + 'abc' 1 $ python type_coercion.py 2 5.0 3 1267650600228229401496703205376 4 abcabc 5 Traceback (most recent call last): 6 File ".../type_coercion.py", line 4, in <module> 7 print 2 + 'abc' 8 TypeError: unsupported operand type(s) for +: 'int' and 'str'
Type Coercion 1 print 5 * 1.0 2 print 2 ** 100 3 print 2 * 'abc' 4 print 2 + 'abc' 1 $ python type_coercion.py 2 5.0 3 1267650600228229401496703205376 4 abcabc 5 Traceback (most recent call last): 6 File ".../type_coercion.py", line 4, in <module> 7 print 2 + 'abc' 8 TypeError: unsupported operand type(s) for +: 'int' and 'str'
Type Coercion 1 print 5 * 1.0 2 print 2 ** 100 3 print 2 * 'abc' 4 print 2 + 'abc' 1 $ python type_coercion.py 2 5.0 3 1267650600228229401496703205376 4 abcabc 5 Traceback (most recent call last): 6 File ".../type_coercion.py", line 4, in <module> 7 print 2 + 'abc' 8 TypeError: unsupported operand type(s) for +: 'int' and 'str'
There should be one – and preferably only one – way to do it. • Eliminate redundancy • Easier to learn • Easier to remember
Index API 1 a = [ 1, 2, 3 ] 2 print a[0] 3 4 a = (1, 2, 3) 5 print a[0] $ python index_api.py 6 1 7 a = 'abc' 1 8 print a[0] a 9 zero value 10 a = {0:'zero value', 11 1:'one value', 12 } 13 print a[0]
Although that way may not be obvious at first unless you’re Dutch. • GUI • Django • GTK • AppEngine • KDE • Pyramid • Cocoa • Flask • MS • TurboGear Windows s • Tk • Pecan • wxPython • Web
Now is better than never. • ctypes • Jython • IronPython • Python 2 vs. Python 3
Although never is often better than right now. • Language moratorium • Not everything goes into stdlib any more
If the implementation is hard to explain, it’s a bad Python Enhancement idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea – let’s do more of those! Closures
Closures 1 def outer(arg): 2 print 'outer(%s)' % arg 3 4 def inner(val): 5 print 'inner(%s) (arg=%s)' % (val, arg) 6 return val * arg 7 8 return inner 9 10 print 'Creating thrice:' 11 thrice = outer(3) 12 13 print 'nCalling thrice:' 14 print thrice(3) 15 print thrice('a')
Closures 1 def outer(arg): 2 print 'outer(%s)' % arg 3 4 def inner(val): 5 print 'inner(%s) (arg=%s)' % (val, arg) 6 return val * arg 7 8 return inner $ python closure.py 9 Creating thrice: 10 print 'Creating thrice:' outer(3) 11 thrice = outer(3) 12 Calling thrice: 13 print 'nCalling thrice:' inner(3) (arg=3) 14 print thrice(3) 9 15 print thrice('a') inner(a) (arg=3) aaa
References • http://docs.python.org/ • http://www.doughellmann.com/ PyMOTW/ • Learning Python by Mark Lutz • Python Essential Reference by David Beazley • The Python Standard Library by Example by Doug Hellmann • http://us.pycon.org/
Questions?
Image Credits 1. http://www.gaialandscapedesignbackgrounds.net/landscape_design_architecture_background_Zen_Garden_chan3.htm 3. http://www.flickr.com/photos/grabadonut/539151245/sizes/l/in/photostream/ 13. http://www.flickr.com/photos/tobascodagama/3106398602/ 17. http://www.flickr.com/photos/benandliz/11065336/ 21 & 22. http://scipy.org 24. http://www.otherlandtoys.co.uk/russian-matryoshka-nesting-dolls-pieces-p-2126.html 32. http://www.dandelionsummers.com/html/building_block.html 36. http://www.flickr.com/photos/photoinference/2494576240/in/faves-40068198@N04/ 47. http://www.flickr.com/photos/smithsonian/3112472619/in/faves-40068198@N04/ 54. http://www.flickr.com/photos/mherring/3722272868/sizes/l/in/photostream/ 57. http://www.webstaurantstore.com/9-3-4-x-4-1-2-unbleached-natural-coffee-filter-12-cup-1000-cs/121CF12U.html 59. http://brainstorm-services.com/wcu-lit/ambiguity.html 66. http://commons.wikimedia.org/wiki/File:Dutch_windmill.jpg 67. http://www.epicpm.org/epic-blogging 68. http://www.skyscanner.net/news/articles/2009/09/002990-travel-tip-10-get-on-last-and-learn-the-art-of-patience.html 69. http://gizmodo.com/5015735/the-top-10-rube-goldberg-machines-featured-on-film 71. http://www.nutricaplabs.com/capsule-manufacturer.aspx 75. http://wmltech.wordpress.com/2006/12/09/got-questions/

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An Introduction to the Zen of Python

  • 1.
    An Introduction to theZen of Python Doug Hellmann February, 2011
  • 2.
    The Zen ofPython by Tim Peters • Beautiful is better than ugly. • There should be one – and • Explicit is better than implicit. preferably only one – obvious way • Simple is better than complex. to do it. • Complex is better than • Although that way may not be complicated. obvious at first unless you're • Flat is better than nested. Dutch. • Sparse is better than dense. • Now is better than never. • Readability counts. • Although never is often better than • Special cases aren't special *right* now. enough to break the rules. • If the implementation is hard to • Although practicality beats purity. explain, it's a bad idea. • Errors should never pass silently. • If the implementation is easy to • Unless explicitly silenced. explain, it may be a good idea. • In the face of ambiguity, refuse the • Namespaces are one honking temptation to guess. great idea – let's do more of those!
  • 3.
    Beautiful is better thanugly. • Write programs for human readers • Simple expression syntax • Consistent syntax and behavior
  • 4.
    Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
  • 5.
    Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
  • 6.
    Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
  • 7.
    Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
  • 8.
    Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
  • 9.
    Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
  • 10.
    Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
  • 11.
    Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
  • 12.
    Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
  • 13.
    Simple Expressions 1 i = 5 + 5 13 2 14 t = ('a', 1, vals, None) 3 s = 'literal string' 15 t[1] 4 16 5 m = """multi-line 17 d = { 'a':'a value', 6 string 18 'b':'b value', 7 """ 19 } 8 20 d['a'] 9 us = u'üñîçø∂é string' 21 10 22 if a or b: 11 vals = [ 1, 3, 5, 7, ] 23 do_something() 12 vals[0] 24
  • 14.
    Explicit is betterthan implicit. • Boolean type • No “hidden” loop variables • Use “self” to refer to object inside a method
  • 15.
    Boolean Values 1 t = True 2 f = False 3 4 if t: 5 print 't is true' $ python booleans.py 6 t is true 7 if not f: f is false 8 print 'f is false' True 9 False 10 a = t or f 11 b = t and f 12 13 print a 14 print b
  • 16.
    for loops $ python for_loop.py 1 print 'Numbers:' Numbers: 2 0 3 for i in range(5): 1 4 print i 2 5 3 6 print 4 7 print 'Letters:' 8 Letters: 9 for c in [ 'a', 'b', 'c' ]: a 10 print c b c
  • 17.
    Defining a Class 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
  • 18.
    Defining a Class 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
  • 19.
    Defining a Class 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
  • 20.
    Defining a Class 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
  • 21.
    Defining a Class 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
  • 22.
    Simple is betterthan complex. • Garbage collection • print statement • Interactive interpreter for experimentation
  • 23.
    Printing 1 # -*- encoding: utf-8 -*- 2 3 print 'string:', 'string' 4 5 print 'unicode:', u'üñîçø∂é string' 6 7 print 'number:', 5 * 5 8 9 my_list = [ 1, 'a', None ] 10 print 'list:', my_list 11 12 my_dict = { 'a':'a value', 'b':'b value' } 13 print 'dict:', my_dict $ python printing.py string: string unicode: üñîçø∂é string number: 25 list: [1, 'a', None] dict: {'a': 'a value', 'b': 'b value'}
  • 24.
    Printing, with printf-styleFormatting 1 # -*- encoding: utf-8 -*- 2 3 my_list = [ 1, 'a', None ] 4 my_dict = { 'a':'a value', 'b':'b value' } 5 6 args = ('string', u'üñîçø∂é string', 5 * 5, 7 my_list, my_dict) 8 9 print ''' 10 string : %s 11 unicode: %s 12 number : %d $ python printing_with_format.py 13 list : %s 14 dict : %s string : string 15 ''' % args unicode: üñîçø∂é string number : 25 list : [1, 'a', None] dict : {'a': 'a value', 'b': 'b value'}
  • 25.
  • 26.
    Complex is better thancomplicated. • Do complex things in a “Pythonic” way • SciPy and NumPy for advanced science and math
  • 27.
    Interactive Graphic SciPy, NumPy, matplotlib Environment
  • 28.
    NumPy Performance Standard Python NumPy 1 a = range(10000000) 1 import numpy as np 2 b = range(10000000) 2 a = np.arange(10000000) 3 c = [] 3 b = np.arange(10000000) 4 for i in range(len(a)): 4 c = a + b 5 c.append(a[i] + b[i]) 5-10 seconds “nearly instantaneous”
  • 29.
    Flat is betterthan nested. • Wide variety of modules in standard library • Namespace is flat • No need to use long names: com.company.java.blah.blah
  • 30.
    Standard Library Modules:Networking • socket • asyncore • select • asynchat • SocketServer • xmlrpclib • BaseHTTPServer • SimpleXMLRPCServer
  • 31.
    Standard Library Modules:Internet Protocols • urllib • poplib • httplib • imaplib • ftplib • json • smtpd • smtplib
  • 32.
    Standard Library Modules:Command Line • getopt • fileinput • optparse • cmd • argparse • readline
  • 33.
    Standard Library Modules:Parallel Processing • subprocess • multiprocessing • threading • Queue
  • 34.
    Standard Library Modules:Databases • anydbm • shelve • pickle • sqlite3
  • 35.
    Standard Library Modules:Miscellaneous • datetime • struct • grp • ctypes • pwd • xml • profile • tarfile • timeit • csv
  • 36.
    Sparse is betterthan dense. • Standard library stays “shallow” • http://pypi.python.org/ • Allows separate release schedules • pip (“pip installs Python packages”)
  • 37.
    Readability counts. • Whitespacefor block structures • Minimal punctuation • Built-in documentation • Extensive online documentation at http:// docs.python.org/
  • 38.
    Code Blocks 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
  • 39.
    Code Blocks 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
  • 40.
    Minimal Punctuation 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
  • 41.
    Minimal Punctuation 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
  • 42.
    Inline Documentation 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
  • 43.
    Inline Documentation 1 class MyClass(object): 2 """Class documentation. 3 """ 4 5 def __init__(self, arg1, arg2): 6 self.attr1 = arg1 7 self.attr2 = arg2 8 9 def do_something(self): 10 """Does some work and returns a value. 11 """ 12 return self.attr1 * self.attr2 13 14 obj = MyClass(1, 2) 15 print obj.do_something()
  • 44.
    Special cases aren’tspecial enough to break the rules. • Everything is an object • Methods and functions differ by scope • Dynamic typing and runtime attribute lookup • Most “features” in external modules
  • 45.
    String Methods $ python string_methods.py 1 print ' string value '.split() ['string', 'value'] 2 print ' string value '.strip() string value 3 print 'str' in ' string value ' True 4 print 'CHUGALUG'.startswith('C') True 5 print 'CHUGALUG'.endswith('end') False 6 print ':'.join(['a', 'b', 'c']) a:b:c
  • 46.
    String Methods $ python string_methods.py 1 print ' string value '.split() ['string', 'value'] 2 print ' string value '.strip() string value 3 print 'str' in ' string value ' True 4 print 'CHUGALUG'.startswith('C') True 5 print 'CHUGALUG'.endswith('end') False 6 print ':'.join(['a', 'b', 'c']) a:b:c
  • 47.
    String Methods $ python string_methods.py 1 print ' string value '.split() ['string', 'value'] 2 print ' string value '.strip() string value 3 print 'str' in ' string value ' True 4 print 'CHUGALUG'.startswith('C') True 5 print 'CHUGALUG'.endswith('end') False 6 print ':'.join(['a', 'b', 'c']) a:b:c
  • 48.
    String Methods $ python string_methods.py 1 print ' string value '.split() ['string', 'value'] 2 print ' string value '.strip() string value 3 print 'str' in ' string value ' True 4 print 'CHUGALUG'.startswith('C') True 5 print 'CHUGALUG'.endswith('end') False 6 print ':'.join(['a', 'b', 'c']) a:b:c
  • 49.
    String Methods $ python string_methods.py 1 print ' string value '.split() ['string', 'value'] 2 print ' string value '.strip() string value 3 print 'str' in ' string value ' True 4 print 'CHUGALUG'.startswith('C') True 5 print 'CHUGALUG'.endswith('end') False 6 print ':'.join(['a', 'b', 'c']) a:b:c
  • 50.
    String Methods $ python string_methods.py 1 print ' string value '.split() ['string', 'value'] 2 print ' string value '.strip() string value 3 print 'str' in ' string value ' True 4 print 'CHUGALUG'.startswith('C') True 5 print 'CHUGALUG'.endswith('end') False 6 print ':'.join(['a', 'b', 'c']) a:b:c
  • 51.
    String Methods $ python string_methods.py 1 print ' string value '.split() ['string', 'value'] 2 print ' string value '.strip() string value 3 print 'str' in ' string value ' True 4 print 'CHUGALUG'.startswith('C') True 5 print 'CHUGALUG'.endswith('end') False 6 print ':'.join(['a', 'b', 'c']) a:b:c
  • 52.
    Function and MethodScope 1 class MyClass(object): 2 3 def do_something(self, arg1, arg2): 4 return arg1 * arg2 5 6 def my_function(arg1, arg2): 7 return arg1 * arg2
  • 53.
    Dynamic Attribute Lookup 1 class MyClass(object): 2 3 CLASS_ATTR = 'class value' 4 5 def __init__(self, arg): 6 self.attr = arg 7 8 o = MyClass('instance value') 9 print o.attr 10 print o.CLASS_ATTR $ python dynamic_attribute_lookup.py instance value class value
  • 54.
    Dynamic Typing 1 class A(object): 2 def do_something(self): 3 print 'in A' 4 5 class B(object): $ python dynamic_typing.py 6 def do_something(self): in A 7 print 'in B' in B 8 9 def work_on_obj(c): 10 c.do_something() 11 12 a = A() 13 b = B() 14 15 work_on_obj(a) 16 work_on_obj(b)
  • 55.
    Regular Expressions 1 import re 2 3 pattern = 'this' 4 text = 'Does this text match the pattern?' 5 6 match = re.search(pattern, text) 7 8 s = match.start() 9 e = match.end() 10 11 print 'Found :', match.re.pattern 12 print 'In :', match.string 13 print 'Range :', s, '-', e 14 print 'Substring:', text[s:e] $ python using_regex.py Found : this In : Does this text match the pattern? Range : 5 - 9 Substring: this
  • 56.
    Although practicality beats purity. •Multiple Programming Models • OOP • Procedural • Functional
  • 57.
    Procedural 1 def my_function(arg1, arg2): 2 """This function multiplies arg1 by arg2. 3 """ 4 return arg1 * arg2 5 6 results = [] 7 for i in range(5): 8 results.append(my_function(i, i)) 9 10 print results $ python procedural.py [0, 1, 4, 9, 16]
  • 58.
    Functional: List Comprehension 1 results = [ i*i for i in range(5) ] 2 3 print results $ python functional.py [0, 1, 4, 9, 16]
  • 59.
    Functional: List Comprehension 1 results = [ i*i for i in range(5) ] 2 3 print results $ python functional.py [0, 1, 4, 9, 16]
  • 60.
    Functional: List Comprehension 1 results = [ i*i for i in range(5) ] 2 3 print results $ python functional.py [0, 1, 4, 9, 16]
  • 61.
    Functional: List Comprehension 1 results = [ i*i for i in range(5) ] 2 3 print results $ python functional.py [0, 1, 4, 9, 16]
  • 62.
    Functional: Generator Expression 1 results = ( i*i for i in range(5) ) 2 3 print results 4 5 for i in results: 6 print i 1 $ python generator_expr.py 2 <generator object <genexpr> at 0x1004d20a0> 3 0 4 1 5 4 6 9 7 16
  • 63.
    Functional: Generators 1 def gen_values(n): 2 for i in range(n): 3 yield i*i 4 5 print gen_values(5) 6 7 for i in gen_values(5): 8 print i 1 $ python generator.py 2 <generator object gen_values at 0x1004d2050> 3 0 4 1 5 4 6 9 7 16
  • 64.
    Errors should never passsilently. • Exception-based error handling • Tracebacks aid debugging
  • 65.
    Exceptions and Tracebacks 1 def f(n): 2 if n > 0: 3 return n * f(n-1) 4 return 1 5 6 print f('5') $ python traceback.py Traceback (most recent call last): File ".../traceback.py", line 6, in <module> print f('5') File ".../traceback.py", line 3, in f return n * f(n-1) TypeError: unsupported operand type(s) for -: 'str' and 'int'
  • 66.
    Raising Exceptions 1 def f(n): 2 if not isinstance(n, int): 3 raise TypeError('n should be an int') 4 if n > 0: 5 return n * f(n-1) 6 return 1 7 8 print f('5') $ python raising.py Traceback (most recent call last): File ".../raising.py", line 8, in <module> print f('5') File ".../raising.py", line 3, in f raise TypeError('n should be an int') TypeError: n should be an int
  • 67.
    Raising Exceptions 1 def f(n): 2 if not isinstance(n, int): 3 raise TypeError('n should be an int') 4 if n > 0: 5 return n * f(n-1) 6 return 1 7 8 print f('5') $ python raising.py Traceback (most recent call last): File ".../raising.py", line 8, in <module> print f('5') File ".../raising.py", line 3, in f raise TypeError('n should be an int') TypeError: n should be an int
  • 68.
    Unless explicitly silenced. • Catchexceptions with try:except • Process • Convert • Ignore
  • 69.
    Catching Exceptions 1 def f(n): 2 if not isinstance(n, int): 3 raise TypeError('n should be an int') 4 if n > 0: 5 return n * f(n-1) 6 return 1 7 8 try: 9 print f('5') 10 except TypeError, err: 11 print 'Had an error:', err 12 else: 13 print 'No error' 14 finally: 15 print 'Always run' $ python catching.py Had an error: n should be an int Always run
  • 70.
    In the faceof ambiguity, refuse the temptation to guess. • Coerce types only when not surprising • Cannot add strings and numbers • Can multiply them!
  • 71.
    Type Coercion 1 print 5 * 1.0 2 print 2 ** 100 3 print 2 * 'abc' 4 print 2 + 'abc' 1 $ python type_coercion.py 2 5.0 3 1267650600228229401496703205376 4 abcabc 5 Traceback (most recent call last): 6 File ".../type_coercion.py", line 4, in <module> 7 print 2 + 'abc' 8 TypeError: unsupported operand type(s) for +: 'int' and 'str'
  • 72.
    Type Coercion 1 print 5 * 1.0 2 print 2 ** 100 3 print 2 * 'abc' 4 print 2 + 'abc' 1 $ python type_coercion.py 2 5.0 3 1267650600228229401496703205376 4 abcabc 5 Traceback (most recent call last): 6 File ".../type_coercion.py", line 4, in <module> 7 print 2 + 'abc' 8 TypeError: unsupported operand type(s) for +: 'int' and 'str'
  • 73.
    Type Coercion 1 print 5 * 1.0 2 print 2 ** 100 3 print 2 * 'abc' 4 print 2 + 'abc' 1 $ python type_coercion.py 2 5.0 3 1267650600228229401496703205376 4 abcabc 5 Traceback (most recent call last): 6 File ".../type_coercion.py", line 4, in <module> 7 print 2 + 'abc' 8 TypeError: unsupported operand type(s) for +: 'int' and 'str'
  • 74.
    Type Coercion 1 print 5 * 1.0 2 print 2 ** 100 3 print 2 * 'abc' 4 print 2 + 'abc' 1 $ python type_coercion.py 2 5.0 3 1267650600228229401496703205376 4 abcabc 5 Traceback (most recent call last): 6 File ".../type_coercion.py", line 4, in <module> 7 print 2 + 'abc' 8 TypeError: unsupported operand type(s) for +: 'int' and 'str'
  • 75.
    Type Coercion 1 print 5 * 1.0 2 print 2 ** 100 3 print 2 * 'abc' 4 print 2 + 'abc' 1 $ python type_coercion.py 2 5.0 3 1267650600228229401496703205376 4 abcabc 5 Traceback (most recent call last): 6 File ".../type_coercion.py", line 4, in <module> 7 print 2 + 'abc' 8 TypeError: unsupported operand type(s) for +: 'int' and 'str'
  • 76.
    There should beone – and preferably only one – way to do it. • Eliminate redundancy • Easier to learn • Easier to remember
  • 77.
    Index API 1 a = [ 1, 2, 3 ] 2 print a[0] 3 4 a = (1, 2, 3) 5 print a[0] $ python index_api.py 6 1 7 a = 'abc' 1 8 print a[0] a 9 zero value 10 a = {0:'zero value', 11 1:'one value', 12 } 13 print a[0]
  • 78.
    Although that waymay not be obvious at first unless you’re Dutch. • GUI • Django • GTK • AppEngine • KDE • Pyramid • Cocoa • Flask • MS • TurboGear Windows s • Tk • Pecan • wxPython • Web
  • 79.
    Now is betterthan never. • ctypes • Jython • IronPython • Python 2 vs. Python 3
  • 80.
    Although never isoften better than right now. • Language moratorium • Not everything goes into stdlib any more
  • 81.
    If the implementationis hard to explain, it’s a bad Python Enhancement idea.
  • 82.
    If the implementationis easy to explain, it may be a good idea.
  • 83.
    Namespaces are one honkinggreat idea – let’s do more of those! Closures
  • 84.
    Closures 1 def outer(arg): 2 print 'outer(%s)' % arg 3 4 def inner(val): 5 print 'inner(%s) (arg=%s)' % (val, arg) 6 return val * arg 7 8 return inner 9 10 print 'Creating thrice:' 11 thrice = outer(3) 12 13 print 'nCalling thrice:' 14 print thrice(3) 15 print thrice('a')
  • 85.
    Closures 1 def outer(arg): 2 print 'outer(%s)' % arg 3 4 def inner(val): 5 print 'inner(%s) (arg=%s)' % (val, arg) 6 return val * arg 7 8 return inner $ python closure.py 9 Creating thrice: 10 print 'Creating thrice:' outer(3) 11 thrice = outer(3) 12 Calling thrice: 13 print 'nCalling thrice:' inner(3) (arg=3) 14 print thrice(3) 9 15 print thrice('a') inner(a) (arg=3) aaa
  • 86.
    References • http://docs.python.org/ • http://www.doughellmann.com/ PyMOTW/ • Learning Python by Mark Lutz • Python Essential Reference by David Beazley • The Python Standard Library by Example by Doug Hellmann • http://us.pycon.org/
  • 87.
  • 88.
    Image Credits 1. http://www.gaialandscapedesignbackgrounds.net/landscape_design_architecture_background_Zen_Garden_chan3.htm 3.http://www.flickr.com/photos/grabadonut/539151245/sizes/l/in/photostream/ 13. http://www.flickr.com/photos/tobascodagama/3106398602/ 17. http://www.flickr.com/photos/benandliz/11065336/ 21 & 22. http://scipy.org 24. http://www.otherlandtoys.co.uk/russian-matryoshka-nesting-dolls-pieces-p-2126.html 32. http://www.dandelionsummers.com/html/building_block.html 36. http://www.flickr.com/photos/photoinference/2494576240/in/faves-40068198@N04/ 47. http://www.flickr.com/photos/smithsonian/3112472619/in/faves-40068198@N04/ 54. http://www.flickr.com/photos/mherring/3722272868/sizes/l/in/photostream/ 57. http://www.webstaurantstore.com/9-3-4-x-4-1-2-unbleached-natural-coffee-filter-12-cup-1000-cs/121CF12U.html 59. http://brainstorm-services.com/wcu-lit/ambiguity.html 66. http://commons.wikimedia.org/wiki/File:Dutch_windmill.jpg 67. http://www.epicpm.org/epic-blogging 68. http://www.skyscanner.net/news/articles/2009/09/002990-travel-tip-10-get-on-last-and-learn-the-art-of-patience.html 69. http://gizmodo.com/5015735/the-top-10-rube-goldberg-machines-featured-on-film 71. http://www.nutricaplabs.com/capsule-manufacturer.aspx 75. http://wmltech.wordpress.com/2006/12/09/got-questions/