CODEWITHSUNDEEP
pythonfor-looptuplesiterable-unpacking
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Reputation: 558

Unpacking a sequence structure using a `for` loop statement in Python

I am having a problem understanding why the following cannot be unpacked in the header line of a for loop.

Given that:

>>> (a,b),c = (1,2),3
>>> a
1
>>> b
2
>>> c
3

Why when I then do this do I receive an error?:

for (a,b),c in [(1,2),3]:
...    print(a,b,c)

I am aware of how to get it working, as the following gives me the desired result:

>>> for (a,b),c in [((1,2),3)]:
...    print(a,b,c)
1 2 3

But why is an extra parenthesis enclosing the original object in the header line of the for loop required to achieve this output?

Given that any sequence can be unpacked [(1,2), 3] is syntactically correct as a list sequence, and should theoretically be able to be assigned to the target variables (a,b),c. So why is this not the case and why do I have to enclose the sequence with an additional parenthesis?

Some clarity around this would be greatly appreciated.

Upvotes: 2

Views: 278

Answers (1)

DisappointedByUnaccountableMod
DisappointedByUnaccountableMod

Reputation: 6826

The for loop is iterating over the list entries, so the first time it tries to assign (1,2) to (a,b),c, which won't work. Adding the extra parentheses converts (1,2),3 into a tuple which is a single list entry ((1,2),3) to assign to (a,b),c the first time the for loop iterates, which works.

Try:

for v in [(1,2),3]:
   print v

And you will see that v is assigned (1,2), then 3.

Upvotes: 4

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