# How are lambdas useful? [closed]

Are you talking about lambda expressions? Like

``````lambda x: x**2 + 2*x - 5
``````

Those things are actually quite useful. Python supports a style of programming called functional programming where you can pass functions to other functions to do stuff. Example:

``````mult3 = filter(lambda x: x % 3 == 0, [1, 2, 3, 4, 5, 6, 7, 8, 9])
``````

sets `mult3` to `[3, 6, 9]`, those elements of the original list that are multiples of 3. This is shorter (and, one could argue, clearer) than

``````def filterfunc(x):
return x % 3 == 0
mult3 = filter(filterfunc, [1, 2, 3, 4, 5, 6, 7, 8, 9])
``````

Of course, in this particular case, you could do the same thing as a list comprehension:

``````mult3 = [x for x in [1, 2, 3, 4, 5, 6, 7, 8, 9] if x % 3 == 0]
``````

(or even as `range(3,10,3)`), but there are many other, more sophisticated use cases where you can’t use a list comprehension and a lambda function may be the shortest way to write something out.

• Returning a function from another function

``````  >>> def transform(n):
...     return lambda x: x + n
...
>>> f = transform(3)
>>> f(4)
7
``````

This is often used to create function wrappers, such as Python’s decorators.

• Combining elements of an iterable sequence with `reduce()`

``````  >>> reduce(lambda a, b: '{}, {}'.format(a, b), [1, 2, 3, 4, 5, 6, 7, 8, 9])
'1, 2, 3, 4, 5, 6, 7, 8, 9'
``````
• Sorting by an alternate key

``````  >>> sorted([1, 2, 3, 4, 5, 6, 7, 8, 9], key=lambda x: abs(5-x))
[5, 4, 6, 3, 7, 2, 8, 1, 9]
``````

I use lambda functions on a regular basis. It took me a while to get used to them, but eventually I came to understand that they’re a very valuable part of the language.