As reported by several users in #148, version 0.10.0 changed how some .env files are parsed in a breaking way, especially with regards to escapes. I think some of those changes were necessary, but some other may have been unexpected.

Experiment

It was unclear what exactly broke and whether Python-dotenv is consistent with other parsers, so I ran an experiment to compare versions and packages.

The scripts to generate these tables are in bbc2/dotenv-parser-comparisons. I may update them if I find new interesting behavior.

Basic

foo=ab

Escaped z

foo=a\zb

Escaped and single-quoted z

foo='a\zb'

Escaped and double-quoted z

foo="a\zb"

Escaped n

foo=a\nb

Escaped and single-quoted n

foo='a\nb'

Escaped and double-quoted n

foo="a\nb"

Quoted newline

foo="a
b"

Non-escaped space

foo=a b

Non-escaped #

foo=a#b

Non-escaped spaced #

foo=a #b

Escaped #

foo="a#b"

UTF-8

foo=é

Quoted UTF-8

foo="é"

Conclusion

1. Non-quoted escapes (valid or invalid): All up-to-date parsers except Bash do the same thing. Bash removes the \. 0.9.1 added a \, which was obviously incorrect.
2. Single-quoted invalid escapes: Everything is fine.
3. Single-quoted valid escapes: 0.10.1 is the only parser that interprets them as control characters.
4. Double-quoted invalid escapes: It's fine except for Ruby.
5. Double-quoted valid escapes: All up-to-date parsers except Bash do the same thing. Bash and 0.9.1 keep the original characters instead of interpreting them as control characters.
6. Pound sign #: Interpreted as a comment delimiter since 0.10.0 if unquoted, even if there is no whitespace preceding it. When quoted or prefixed with whitespace, everything is fine except for JavaScript.
7. Non-quoted UTF-8: Fixed in 0.10.0. When quoted, everything is fine.
8. Non-escaped space: Only Bash ignores everything after it (or treats the rest as a command). Other parsers include everything until the end of the line.

My opinion:

• (2, 4, 7) are OK.
• (1, 5, 8) are where Bash differs from other parsers. It isn't obvious what we should do.
• (3, 6) are where python-dotenv 0.10.0 is quite obviously broken and should be fixed.