问题
Does anybody know the reasoning as to why the unary (*
) operator cannot be used in an expression involving iterators/lists/tuples?
Why is it only limited to function unpacking? or am I wrong in thinking that?
For example:
>>> [1,2,3, *[4,5,6]]
File "<stdin>", line 1
[1,2,3, *[4,5,6]]
^
SyntaxError: invalid syntax
Why doesn't the *
operator:
[1, 2, 3, *[4, 5, 6]] give [1, 2, 3, 4, 5, 6]
whereas when the *
operator is used with a function call it does expand:
f(*[4, 5, 6]) is equivalent to f(4, 5, 6)
There is a similarity between the +
and the *
when using lists but not when extending a list with another type.
For example:
# This works
gen = (x for x in range(10))
def hello(*args):
print args
hello(*gen)
# but this does not work
[] + gen
TypeError: can only concatenate list (not "generator") to list
回答1:
Unpacking in list, dict, set, and tuple literals has been added in Python 3.5
, as described in PEP 448:
Python 3.5.0 (v3.5.0:374f501f4567, Sep 13 2015, 02:27:37) on Windows (64 bits).
>>> [1, 2, 3, *[4, 5, 6]]
[1, 2, 3, 4, 5, 6]
Here are some explanations for the rationale behind this change. Note that this does not make *[1, 2, 3]
equivalent to 1, 2, 3
in all contexts. Python's syntax is not intended to work that way.
回答2:
Asterix *
isn't simply unary operator, it's argument-unpacking operator for functions definitions and functions calls.
So *
supposed to be used only to work with function params and not with lists, tuples etc.
NOTE: starting from python3.5, *
could be used not only with functions params, @B. M's answer greatly describes that change in python.
If you need to concat lists use concatenation instead list1 + list2
to get desired result.
To concatenate list and generator simply pass generator
to list
type object, prior concatenating with another list:
gen = (x for x in range(10))
[] + list(gen)
回答3:
This is not supported. Python 3 gives a better message (though Python 2 does not support *
in the left part of an assignment, afaik):
Python 3.4.3+ (default, Oct 14 2015, 16:03:50)
>>> [1,2,3, *[4,5,6]]
File "<stdin>", line 1
SyntaxError: can use starred expression only as assignment target
>>>
f(*[4,5,6])
is equivalent tof(4,5,6)
Function argument unfolding is a special case.
来源:https://stackoverflow.com/questions/34766677/python-splat-unpack-operator-in-python-cannot-be-used-in-an-expression