Speaker Trey Hunner
Time 2017-08-05 10:45
Conference PyCon Au 2017

Comprehensible Comprehensions


  • Know how for loops work.
  • You are not a fan of list comprehensions.


  • anything you can iterate over.


  • lazy iterable.

List comprehensions

  • Turn one list into another list.
  • Making new list into old list more common in Python.
  • Often copy old list to new list to avoid mutating old list.
  • Contains some information as for loop.
  • Are more specific about intent of code then for loop.


  • Makes readable for loops unreadable.

    • Comprehensions often written on one line. Very condensed. No white space.
    • Operation, Loop, Condition. Don’t have to be on one line:

      .. code-block:: python

      squared_odds = [ n ** 2 for n in numbers if n %2 == 1 ]

    • Should always start by writing comprehension on multiple lines.

Generator expressions

  • If you are only going to loop over result once, you should use a generator expression instead of a list comprehension.

    .. code-block:: python

    squared_numbers = (n ** 2 for n in numbers)

  • Generators are lazy, only generate on demand.
  • Generators are one time only. Can only loop over once. Single use.

When not to use comprehensions

  • Only for turning one list into another list.
  • Do not have side effects, e.g. print, in comprehension.
  • When dict/list constructors can be used instead.
  • When not changing or filtering anything.