Python lambda with if but without else

I found that filter provided exactly what I was looking for in python 2:

>>> data = [1, 2, 5, 10, -1]
>>> filter(lambda x: x < 3, data)
[1, 2, -1]

The implementation is different in 2.x and 3.x: while 2.x provides a list, 3.x provides an iterator. Using a list comprehension might make for a cleaner use in 3.x:

>>> data = [1, 2, 5, 10, -1]
>>> [filter(lambda x: x < 3, data)]
[1, 2, -1]

A lambda, like any function, must have a return value.

lambda x: x if (x<3) does not work because it does not specify what to return if not x<3. By default functions return None, so you could do

lambda x: x if (x<3) else None

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But perhaps what you are looking for is a list comprehension with an if condition. For example:

In [21]: data = [1, 2, 5, 10, -1]

In [22]: [x for x in data if x < 3]
Out[22]: [1, 2, -1]