Python : Map function with none type list as parameter

Question

I want to pass a list of None in a map function but it doesn't work.

a = ['azerty','uiop']
b = ['qsdfg','hjklm']
c = ['wxc','vbn']
d = None

def func1(*y):
    print 'y:',y

map((lambda *x: func1(*x)), a,b,c,d)

I have this message error:

TypeError: argument 5 to map() must support iteration.

Answer 1

Replace None with an empty list:

map(func1, a or [], b or [], c or [], d or [])

or filter the lists:

map(func1, *filter(None, (a, b, c, d)))

The filter() call removes d from the list altogether, while the first option gives you None values to your function call.

I removed the lambda, it is redundant here.

With the or [] option, the 4th argument is None :

>>> map(func1, a or [], b or [], c or [], d or [])
y: ('azerty', 'qsdfg', 'wxc', None)
y: ('uiop', 'hjklm', 'vbn', None)
[None, None]

Filtering results in 3 arguments to func1 :

>>> map(func1, *filter(None, (a, b, c, d)))
y: ('azerty', 'qsdfg', 'wxc')
y: ('uiop', 'hjklm', 'vbn')
[None, None]

You could use itertools.starmap() as well, but that gets a little verbose:

>>> list(starmap(func1, zip(*filter(None, (a, b, c, d)))))
y: ('azerty', 'qsdfg', 'wxc')
y: ('uiop', 'hjklm', 'vbn')
[None, None]

Answer 2

输入第二个参数以映射列表或元组：

map((lambda *x): func1(*x)), (a,b,c,d))

Answer 3

The error message pretty much says it all: None is not iterable. Arguments to map should be iterable:

map(func, *iterables) --> map object

Make an iterator that computes the function using arguments from
each of the iterables.  Stops when the shortest iterable is exhausted.

Depending on what you want to achieve, you can:

• change None to an empty list;
• map your function on a list of [a, b, c, d]

Also note that you can map func1 directly, without a lambda:

map(func1, *iterables)

Answer 4

第二个参数d应该是SEQUENCE，使其成为列表或元组。