# Are you talking about [lambda functions][1]? 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.
#
# [1]: https://docs.python.org/3.5/tutorial/controlflow.htmllambda-
# expressions
#
# [David Z] [so/q/890128] [cc by-sa 3.0]
$
cheat.sh