CODEWITHSUNDEEP
pythonpython-3.x
amiref
amiref

Reputation: 3441

Why can't I use multiple *vars in tuple unpacking on the left-hand side of an assignment?

I am trying to make sense of starred expressions in python. When I use it in python functions, it allows to call functions with different number of arguments:

def my_sum(*args):
    results = 0
    for x in args:
        results += x
    return results

print(my_sum(1,2,3))
>>6

print(my_sum(1,2,3,4,5,6,7))]
>>28

However when I use it in an assignment, it works like this:

a, b, *c, d = [1,2,3,4,5,6,7,8,9,10]
print(a,b,c,d)

>>1 2 [3, 4, 5, 6, 7, 8, 9] 10

*a, b, *c, d = [1,2,3,4,5,6,7,8,9,10]
print(a,b,c,d)

*a, b, *c, d = [1,2,3,4,5,6,7,8,9,10]
^
SyntaxError: multiple starred expressions in assignment

Can someone explain to me what is happening behind this assignment that doesn't allow multiple starred expressions?

Upvotes: 1

Views: 347

Answers (1)

Charles Duffy
Charles Duffy

Reputation: 296049

The language doesn't allow this because it would be ambiguous, and allowing ambiguities is contrary to the Zen of Python:

In the face of ambiguity, refuse the temptation to guess.

Let's say you run:

*a, b, *c, d = [1,2,3,4,5,6,7,8,9,10]

One way to interpret that would be:

a = [1,2,3,4,5,6,7]
b = 8
c = [9]
d = 10

Another would be:

a = [1]
b = 2
c = [3,4,5,6,7,8,9]
d = 10

Yet another would be:

a = []
b = 1
c = [2,3,4,5,6,7,8,9]
d = 10

Python refuses to guess: It simply declares the code invalid.

Upvotes: 2

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