I was writing some lambda functions and couldn't figure this out. Is there a way to have something like lambda x: x if (x<3) in python? As lambda a,b: a if (a > b) else b works ok. So far lambda x: x < 3 and x or None seems to be the closest i have found.
Add a comment
|
5 Answers
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
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]
3 Comments
root
@ ubuntu -- thank you. Not sure why I assumed it should return None without specifying it.
unutbu
That's a reasonable assumption since functions return None by default. The important thing to remember is that what follows
lambda x: must be an expression, not a statement.
Jon Clements
@root I sometimes find it handy to think of a
lambda as though it's actually written as def lambda(x, y): return ...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]
3 Comments
wjandrea
The implementation is not the same in both 2 and 3. In 2 it returns a list, in 3 it returns an iterator, and to get it into a list, you need to convert it:
list(filter(lambda, data)). Either way, it's generally considered easier to read a comprehension with an if clause like in unutbu's answer.
im_just_here_to_learn
Added your input about python 3, thank you! I suppose the stylistic changes are a bit about personal preference. Lambdas feel a bit natural to me due to using functional programming languages. When I read them, something like
x: x < 3 comes across as less repetitive/more elegant than something like x for x in data if x [...].wjandrea
Welcome! But your Python3 implementation is incorrect. You get a filter object inside a list. You need to convert it:
list(filter(lambda, data)) or use a splat: [*filter(lambda x: x < 3, data)]. It's also not a list comprehension.What's wrong with lambda x: x if x < 3 else None?
6 Comments
zinking
my understanding is that list comprehension is more appropriate in this scenario
user4815162342
The question doesn't mention iteration, so I'm not sure what you mean. Can you elaborate?
Mardini
x will have a value of "None" in case of else
user4815162342
@Mardini No, but the
lambda will return None in that case. It has to return something.Mardini
@user4815162342 This is exactly what I meant, lambda will return "None"
|
You can always try to invoke 'filter' for conditional checks. Fundamentally, map() has to work on every occurrence of the iterables, so it cannot pick and choose. But filter may help narrow down the choices. For example, I create a list from 1 to 19 but want to create a tuple of squares of only even numbers.
x = list(range(1,20))
y = tuple(map(lambda n: n**2, filter(lambda n: n%2==0,x)))
print (y)
1 Comment
Sourabh Desai
The answers I was looking for, Thanks. If somehow you can manage to add a 'reduce' statement here and boom we can have one liner functional programming tutorial.
You can use ellipsis ... to fill else statement
lambda x: x if (x<3) else ...
Note it does not work with pass.
