Python Pandas.pptx

Introduction to Pandas’
DataFrame
Ms.Sujaya S
Assistant Professor
Department of CSE
KIT-Kalaignarkarunanidhi Institute of Technology

Pandas First Steps: install and import
• Pandas is an easy package to install. Open up your terminal program (shell or cmd)
and install it using either of the following commands:
• For jupyter notebook users, you can run this cell:
• The ! at the beginning runs cells as if they were in a terminal.
• To import pandas we usually import it with a shorter name since it's used so much:
$ conda install pandas
OR
$ pip install pandas
!pip install pandas
import pandas as pd

pandas: Data Table Representation

Core components of pandas: Series & DataFrames
• The primary two components of pandas are the Series and DataFrame.
• Series is essentially a column, and
• DataFrame is a multi-dimensional table made up of a collection of Series.
• DataFrames and Series are quite similar in that many operations that you can do with one you can do
with the other, such as filling in null values and calculating the mean.
• A Data frame is a two-dimensional data structure, i.e., data is aligned in a tabular fashion in rows and columns.
• Features of DataFrame
• Potentially columns are of different types
• Size – Mutable
• Labeled axes (rows and columns)
• Can Perform Arithmetic operations on rows
and columns
rows
columns

Types of Data Structure in Pandas
Data Structure Dimensions Description
Series 1 1D labeled homogeneous array with immutable size
Data Frames 2
General 2D labeled, size mutable tabular structure
with potentially heterogeneously typed columns.
Panel 3 General 3D labeled, size mutable array.
• Series & DataFrame
• Series is a one-dimensional array (1D Array) like structure with homogeneous data.
• DataFrame is a two-dimensional array (2D Array) with heterogeneous data.
• Panel
• Panel is a three-dimensional data structure (3D Array) with heterogeneous data.
• It is hard to represent the panel in graphical representation.
• But a panel can be illustrated as a container of DataFrame

pandas.DataFrame
• data: data takes various forms like ndarray, series, map, lists, dict, constants and also another DataFrame.
• index: For the row labels, that are to be used for the resulting frame, Optional, Default is np.arrange(n)if no index is passed.
• columns: For column labels, the optional default syntax is - np.arrange(n). This is only true if no index is passed.
• dtype: Data type of each column.
• copy: This command (or whatever it is) is used for copying of data, if the default is False.
pandas.DataFrame(data, index , columns , dtype , copy )
• Create DataFrame
• A pandas DataFrame can be created using various inputs like −
• Lists
• dict
• Series
• Numpy ndarrays
• Another DataFrame

Creating a DataFrame from scratch

Creating a DataFrame from scratch
• There are many ways to create a DataFrame from scratch, but a great option is to just use a
simple dict. But first you must import pandas.
• Let's say we have a fruit stand that sells apples and oranges. We want to have a column for each
fruit and a row for each customer purchase. To organize this as a dictionary for pandas we could
do something like:
• And then pass it to the pandas DataFrame constructor:
df = pd.DataFrame(data)
import pandas as pd
data = { 'apples':[3, 2, 0, 1] , 'oranges':[0, 3, 7, 2] }

How did that work?
• Each (key, value) item in data corresponds to a column in the resulting DataFrame.
• The Index of this DataFrame was given to us on creation as the numbers 0-3, but we could also
create our own when we initialize the DataFrame.
• E.g. if you want to have customer names as the index:
• So now we could locate a customer's
order by using their names:
df = pd.DataFrame(data, index=['Ahmad', 'Ali', 'Rashed', 'Hamza'])
df.loc['Ali']

pandas.DataFrame.from_dict
pandas.DataFrame.from_dict(data, orient='columns', dtype=None, columns=None)
• data : dict
• Of the form {field:array-like} or {field:dict}.
• orient : {‘columns’, ‘index’}, default ‘columns’
• The “orientation” of the data.
• If the keys of the passed dict should be the columns of the resulting DataFrame, pass ‘columns’ (default).
• Otherwise if the keys should be rows, pass ‘index’.
• dtype : dtype, default None
• Data type to force, otherwise infer.
• columns : list, default None
• Column labels to use when orient='index'. Raises a ValueError if used with
orient='columns'.
https://pandas.pydata.org/pandas-docs/version/0.23/generated/pandas.DataFrame.from_dict.html

pandas’ orient keyword
data = {'col_1':[3, 2, 1, 0], 'col_2':['a','b','c','d']}
pd.DataFrame.from_dict(data)
data = {'row_1':[3, 2, 1, 0], 'row_2':['a','b','c','d']}
pd.DataFrame.from_dict(data,
orient='index')
data = {'row_1':[3, 2, 1, 0], 'row_2':['a','b','c','d']}
pd.DataFrame.from_dict(data,
orient = 'index',
columns = ['A','B','C','D'])

Loading a DataFrame from files

Reading data from CSVs
• With CSV files, all you need is a single line to load in the data:
df = pd.read_csv('dataset.csv')
• CSVs don't have indexes like our DataFrames, so all we need to do is just designate the
index_col when reading:
df = pd.read_csv('dataset.csv', index_col=0)
• Note: here we're setting the index to be column zero.

Reading data from JSON
• If you have a JSON file — which is essentially a stored Python dict — pandas can read this just as
easily:
df = pd.read_json('dataset.json')
• Notice this time our index came with us correctly since using JSON allowed indexes to work
through nesting.
• Pandas will try to figure out how to create a DataFrame by analyzing structure of your JSON, and
sometimes it doesn't get it right.
• Often you'll need to set the orient keyword argument depending on the structure

Example #1:Reading data from JSON

Example #2: Reading data from JSON

Example #3: Reading data from JSON

Converting back to a CSV or JSON
• So after extensive work on cleaning your data, you’re now ready to save it as a file of your choice.
Similar to the ways we read in data, pandas provides intuitive commands to save it:
• When we save JSON and CSV files, all we have to input into those functions is our desired
filename with the appropriate file extension.
df.to_csv('new_dataset.csv')
df.to_json('new_dataset.json')
df.to_sql('new_dataset', con)
Reference: https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.to_json.html

Most important DataFrame
operations
• DataFrames possess hundreds of methods and other operations that are crucial to any analysis.
• As a beginner, you should know the operations that:
• that perform simple transformations of your data and those
• that provide fundamental statistical analysis on your data.

Loading dataset
• We're loading this dataset from a CSV and designating the movie titles to be our
index.
movies_df = pd.read_csv("movies.csv", index_col="title")
https://grouplens.org/datasets/movielens/

Viewing your data
• The first thing to do when opening a new dataset is print out a few rows to keep as a visual
reference. We accomplish this with .head():
• .head() outputs the first five rows of your DataFrame by default, but we could also pass a number
as well: movies_df.head(10) would output the top ten rows, for example.
• To see the last five rows use .tail() that also accepts a number, and in this case we printing
the bottom two rows.:
movies_df.head()
movies_df.tail(2)

Getting info about your data
• .info() should be one of the very first commands you run after loading your data
• .info() provides the essential details about your dataset, such as the number of rows and
columns, the number of non-null values, what type of data is in each column, and how much
memory your DataFrame is using.
movies_df.info()
movies_df.shape

Handling duplicates
• This dataset does not have duplicate rows, but it is always important to verify you aren't
aggregating duplicate rows.
• To demonstrate, let's simply just double up our movies DataFrame by appending it to itself:
• Using append() will return a copy without affecting the original DataFrame. We are capturing
this copy in temp so we aren't working with the real data.
• Notice call .shape quickly proves our DataFrame rows have doubled.
temp_df = movies_df.append(movies_df)
temp_df.shape
Now we can try dropping duplicates:
temp_df = temp_df.drop_duplicates()
temp_df.shape

Handling duplicates
• Just like append(), the drop_duplicates() method will also return a copy of your
DataFrame, but this time with duplicates removed. Calling .shape confirms we're back to the
1000 rows of our original dataset.
• It's a little verbose to keep assigning DataFrames to the same variable like in this example. For this
reason, pandas has the inplace keyword argument on many of its methods. Using
inplace=True will modify the DataFrame object in place:
• Another important argument for drop_duplicates() is keep, which has three possible
options:
• first: (default) Drop duplicates except for the first occurrence.
• last: Drop duplicates except for the last occurrence.
• False: Drop all duplicates.
temp_df.drop_duplicates(inplace=True)
https://www.learndatasci.com/tutorials/python-pandas-tutorial-complete-introduction-for-beginners/

Understanding your variables
• Using .describe() on an entire DataFrame we can get a summary of the distribution of
continuous variables:
• .describe() can also be used on a categorical variable to get the count of rows, unique
count of categories, top category, and freq of top category:
• This tells us that the genre column has 207 unique values, the top value is Action/Adventure/Sci-
Fi, which shows up 50 times (freq).
movies_df.describe()
movies_df['genre'].describe()

More Examples
import pandas as pd
data = [1,2,3,10,20,30]
print(df)
import pandas as pd
data = {'Name' : ['AA', 'BB'], 'Age': [30,45]}
print(df)

More Examples
import pandas as pd
data = [{'a': 1, 'b': 2},{'a': 5, 'b': 10, 'c': 20}]
print(df)
import pandas as pd
data = [{'a': 1, 'b': 2},{'a': 5, 'b': 10, 'c': 20}]
df = pd.DataFrame(data, index=['first', 'second'])
print(df)
a b c
0 1 2 NaN
1 5 10 20.0
a b c
first 1 2 NaN
second 5 10 20.0

More Examples
import pandas as pd
data = [{'a': 1, 'b': 2},{'a': 5, 'b': 10, 'c': 20}]
#With two column indices, values same as dictionary keys
df1 = pd.DataFrame(data,index=['first','second'],columns=['a','b'])
#With two column indices with one index with other name
df2 = pd.DataFrame(data,index=['first','second'],columns=['a','b1'])
print(df1)
print('...........')
print(df2)
E.g. This shows how to create a DataFrame with a list of dictionaries, row indices, and column indices.
a b
first 1 2
second 5 10
...........
a b1
first 1 NaN
second 5 NaN

More Examples:
Create a DataFrame from Dict of Series
import pandas as pd
d = {'one' : pd.Series([1, 2, 3] , index=['a', 'b', 'c']),
'two' : pd.Series([1,2, 3, 4], index=['a', 'b', 'c', 'd'])
}
df = pd.DataFrame(d)
print(df)
one two
a 1.0 1
b 2.0 2
c 3.0 3
d NaN 4

More Examples: Column Addition
import pandas as pd
d = {'one':pd.Series([1,2,3], index=['a','b','c']),
'two':pd.Series([1,2,3,4], index=['a','b','c','d'])
}
# Adding a new column to an existing DataFrame object
# with column label by passing new series
print("Adding a new column by passing as Series:")
df['three'] = pd.Series([10,20,30],index=['a','b','c'])
print(df)
print("Adding a column using an existing columns in
DataFrame:")
df['four'] = df['one']+df['three']
print(df)
Adding a column using Series:
one two three
a 1.0 1 10.0
b 2.0 2 20.0
c 3.0 3 30.0
d NaN 4 NaN
Adding a column using columns:
one two three four
a 1.0 1 10.0 11.0
b 2.0 2 20.0 22.0
c 3.0 3 30.0 33.0
d NaN 4 NaN NaN

More Examples: Column Deletion
# Using the previous DataFrame, we will delete a column
# using del function
import pandas as pd
d = {'one' : pd.Series([1, 2, 3], index=['a', 'b', 'c']),
'two' : pd.Series([1, 2, 3, 4], index=['a', 'b', 'c', 'd']),
'three' : pd.Series([10,20,30], index=['a','b','c'])
}
print ("Our dataframe is:")
print(df)
# using del function
print("Deleting the first column using DEL function:")
del df['one']
print(df)
# using pop function
print("Deleting another column using POP function:")
df.pop('two')
print(df)
Our dataframe is:
one two three
a 1.0 1 10.0
b 2.0 2 20.0
c 3.0 3 30.0
d NaN 4 NaN
Deleting the first column:
two three
a 1 10.0
b 2 20.0
c 3 30.0
d 4 NaN
Deleting another column:
a 10.0
b 20.0
c 30.0
d NaN

More Examples: Slicing in DataFrames
import pandas as pd
'two' : pd.Series([1, 2, 3, 4], index=['a', 'b', 'c','d'])
}
print(df[2:4])
one two
c 3.0 3
d NaN 4

More Examples: Addition of rows
import pandas as pd
'two' : pd.Series([1, 2, 3, 4], index=['a', 'b', 'c','d'])
}
print(df)
df2 = pd.DataFrame([[5,6], [7,8]], columns = ['a', 'b'])
df = df.append(df2 )
print(df)
one two
a 1.0 1
b 2.0 2
c 3.0 3
d NaN 4
one two a b
a 1.0 1.0 NaN NaN
b 2.0 2.0 NaN NaN
c 3.0 3.0 NaN NaN
d NaN 4.0 NaN NaN
0 NaN NaN 5.0 6.0
1 NaN NaN 7.0 8.0

More Examples: Deletion of rows
import pandas as pd
d = {'one':pd.Series([1, 2, 3], index=['a','b','c']),
'two':pd.Series([1, 2, 3, 4], index=['a','b','c','d'])
}
print(df)
df2 = pd.DataFrame([[5,6], [7,8]], columns = ['a', 'b'])
df = df.append(df2 )
print(df)
df = df.drop(0)
print(df)
one two
a 1.0 1
b 2.0 2
c 3.0 3
d NaN 4
one two a b
a 1.0 1.0 NaN NaN
b 2.0 2.0 NaN NaN
c 3.0 3.0 NaN NaN
d NaN 4.0 NaN NaN
0 NaN NaN 5.0 6.0
1 NaN NaN 7.0 8.0
one two a b
a 1.0 1.0 NaN NaN
b 2.0 2.0 NaN NaN
c 3.0 3.0 NaN NaN
d NaN 4.0 NaN NaN
1 NaN NaN 7.0 8.0

More Examples: Reindexing
import pandas as pd
# Creating the first dataframe
df1 = pd.DataFrame({"A":[1, 5, 3, 4, 2],
"B":[3, 2, 4, 3, 4],
"C":[2, 2, 7, 3, 4],
"D":[4, 3, 6, 12, 7]},
index =["A1", "A2", "A3", "A4", "A5"])
# Creating the second dataframe
df2 = pd.DataFrame({"A":[10, 11, 7, 8, 5],
"B":[21, 5, 32, 4, 6],
"C":[11, 21, 23, 7, 9],
"D":[1, 5, 3, 8, 6]},
index =["A1", "A3", "A4", "A7", "A8"])
# Print the first dataframe
print(df1)
print(df2)
# find matching indexes
df1.reindex_like(df2)
• Pandas
dataframe.reindex_like()
function return an object with
matching indices to myself.
• Any non-matching indexes are filled
with NaN values.

More Examples:
Concatenating Objects (Data Frames)
import pandas as pd
df1 = pd.DataFrame({'Name':['A','B'], 'SSN':[10,20], 'marks':[90, 95] })
df2 = pd.DataFrame({'Name':['B','C'], 'SSN':[25,30], 'marks':[80, 97] })
df3 = pd.concat([df1, df2])
df3

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