Hidden features of Python

What are the lesser-known but useful features of the Python programming language?

Try to limit answers to Python core.

One feature per answer.

Give an example and short description of the feature, not just a link to documentation.

Label the feature using a title as the first line.

Quick links to answers:

Argument Unpacking

Braces

Chaining Comparison Operators

Decorators

Default Argument Gotchas / Dangers of Mutable Default arguments

Descriptors

Dictionary default .get value

Docstring Tests

Ellipsis Slicing Syntax

Enumeration

For/else

Function as iter() argument

Generator expressions

import this

In Place Value Swapping

List stepping

__missing__ items

Multi-line Regex

Named string formatting

Nested list/generator comprehensions

New types at runtime

.pth files

ROT13 Encoding

Regex Debugging

Sending to Generators

Tab Completion in Interactive Interpreter

Ternary Expression

try/except/else

Unpacking+ print() function

with statement

---

Chaining comparison operators:

>>> x = 5
>>> 1 < x < 10
True
>>> 10 < x < 20 
False
>>> x < 10 < x*10 < 100
True
>>> 10 > x <= 9
True
>>> 5 == x > 4
True

In case you're thinking it's doing 1 < x , which comes out as True , and then comparing True < 10 , which is also True , then no, that's really not what happens (see the last example.) It's really translating into 1 < x and x < 10 , and x < 10 and 10 < x * 10 and x*10 < 100 , but with less typing and each term is only evaluated once.

---

Get the python regex parse tree to debug your regex.

Regular expressions are a great feature of python, but debugging them can be a pain, and it's all too easy to get a regex wrong.

Fortunately, python can print the regex parse tree, by passing the undocumented, experimental, hidden flag re.DEBUG (actually, 128) to re.compile .

>>> re.compile("^[font(?:=(?P<size>[-+][0-9]{1,2}))?](.*?)[/font]",
    re.DEBUG)
at at_beginning
literal 91
literal 102
literal 111
literal 110
literal 116
max_repeat 0 1
  subpattern None
    literal 61
    subpattern 1
      in
        literal 45
        literal 43
      max_repeat 1 2
        in
          range (48, 57)
literal 93
subpattern 2
  min_repeat 0 65535
    any None
in
  literal 47
  literal 102
  literal 111
  literal 110
  literal 116

Once you understand the syntax, you can spot your errors. There we can see that I forgot to escape the [] in [/font] .

Of course you can combine it with whatever flags you want, like commented regexes:

>>> re.compile("""
 ^              # start of a line
 [font         # the font tag
 (?:=(?P<size>  # optional [font=+size]
 [-+][0-9]{1,2} # size specification
 ))?
 ]             # end of tag
 (.*?)          # text between the tags
 [/font]      # end of the tag
 """, re.DEBUG|re.VERBOSE|re.DOTALL)

---

enumerate

Wrap an iterable with enumerate and it will yield the item along with its index.

For example:

>>> a = ['a', 'b', 'c', 'd', 'e']
>>> for index, item in enumerate(a): print index, item
...
0 a
1 b
2 c
3 d
4 e
>>>

References:

Python tutorial—looping techniques

Python docs—built-in functions— enumerate

PEP 279

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