如何编写 Inline/Lambda 以追加或不追加到列表

Question

I feel this a noob question but I haven't used lambda functions that much and I couldn't find what I needed online. I wanted to write a lambda function which takes a series as an input, and returns a list without any 'None's in it Basically I want a lambda function for the followinf function:

def get_path(x):
  toRet = []
  for item in x.tolist():
    if item is not None:
      toret.append(item)
  return toRet

Is it possible to get an inline lambda function which does what get_path() does. Yes I do know that I can do the following:

lambda x: get_path(x)

It solves the problem but I would really love to know how to make an inline function for this.

Answer 1

get_path = lambda x: [ elem for elem in x.tolist() if not elem]

Answer 2

You don't even need lambda functions here.

toRet = [item for item in x.tolist() if item is not None]

You can also use filter for example, which looks neater

toRet = list(filter(None, x.tolist()))

Answer 3

As others have pointed out, you can use filter() but be careful of what function you provide for the filter-check.

If function is None , the identity function is assumed, that is, all elements of iterable that are false are removed.

list(filter(None, something)) won't work correctly because it will filter out False-ish values too, not just None . Example:

>>> mylist = [1, 2, 3, 0, None, '', tuple(), 'last']
>>> list(filter(None, mylist))  # not the expected behaviour
[1, 2, 3, 'last']

OP had an explicit if item is not None check, rather than just if item - so the two are not equivalent. Only None needs to be filtered out. To get the correct filtering behaviour, provide an actual check for the function provided to filter:

>>> list(filter(lambda i: i is not None, mylist))
[1, 2, 3, 0, '', (), 'last']

Replace mylist with x.tolist() . And when you put that into a lambda, it gets messy:

get_path = lambda x: list(filter(lambda i: i is not None, x.tolist()))

Instead of all that, the list comprehension option is better:

get_path = lambda x: [i for i in x.tolist() if i is not None]

with an explicit is not None check.