Answer by David Z for How are lambdas useful?

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 == 0mult3 = 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.