if/else in a list comprehension

python list-comprehension
AP257 picture AP257 · Nov 23, 2010 · Viewed 949.1k times · Source

How can I do the following in Python?

row = [unicode(x.strip()) for x in row if x is not None else '']

Essentially:

  1. replace all the Nones with empty strings, and then
  2. carry out a function.

Answer

poke picture poke · Nov 23, 2010

You can totally do that. It's just an ordering issue:

[unicode(x.strip()) if x is not None else '' for x in row]

In general,

[f(x) if condition else g(x) for x in sequence]

And, for list comprehensions with if conditions only,

[f(x) for x in sequence if condition]

Note that this actually uses a different language construct, a conditional expression, which itself is not part of the comprehension syntax, while the if after the for…in is part of list comprehensions and used to filter elements from the source iterable.

Conditional expressions can be used in all kinds of situations where you want to choose between two expression values based on some condition. This does the same as the ternary operator ?: that exists in other languages. For example:

value = 123
print(value, 'is', 'even' if value % 2 == 0 else 'odd')

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