Python string.split more than one value in for loop

Question

Basically this works fine:

>>> x,y = "x=y".split("=")
>>> print x
x

But this gives an error:

>>> for x, y in "x=y".split("="):
...     print x
...

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: need more than 1 value to unpack

I am wondering what the difference is, and how I could fix this for loop.

Solution 1

Split on "=" gives you two values:

"x", "y"

The fact that those values match your variable names is incidental. You could also do:

x,xx = "x=y".split("=")

I suspect what you are likely planning is to take a list:

"foo=bar,blah=boo,etc=something"

And split it, for which you could do:

for x,y in [ (pair.split("=")) for pair in "foo=bar,blah=boo,etc=something".split(",") ]:
    print x,y

BUT! While it works, I think it would be much better to split it into individual steps as it's much more readable:

params = "foo=bar,blah=boo,etc=something"
pair_list = params.split(",")
for pair in pair_list:
    x,y = pair.split("=")
    ...

Solution 2

You could do

for x in "x=y".split("="):
    # ...

What you tried is to iterate over the sequence ["x", "y"], but assign to x, y for each entry of the sequence. That would be equivalent to

 x, y = "x"

for the first iteration, which does not make any sense.

Solution 3

I'm not sure why you would ever want to do this, but if for some reason you would like to use a for loop for this:

>>> for x, y in ["x=y".split("=")]:
...   print x
...   print y
... 
x
y

Comments

[mcot]

Humm.. but I want to assign to "x". Any way to do this without the extra step of saying, x = x[0] inside the for loop?

[Admin]

@user: Using for x in ..., x is the first item of the list split returns (on the first iteration, on the second it's the second item, i.e. "y").

[mcot]

Yep, this is exactly what I intended to do, thanks for the answer.

[Admin]

Not cramming the list comprehension into the loop header is a good idea. Moving the tuple unpacking into the loop and introducing a bogus variable pair is not - just use for x, y in pair_list (ideally, x and y would get more meaningful names as well...).

[Ofer]

@delnan, pair_list looks like ["foo=bar", "blah=boo", "etc=something"], it's not a list of tuples, but a list of strings. There are probably faster ways to process the whole string, but I think it would be a premature optimization. Am I missing something?

[Admin]

No, I somehow thought the pairs were already split (well, I would have suggested pair_list = [raw_pair.split('=') for raw_pair in params.split(',')]).

[Ofer]

@delnan, no problem. BTW, that's effectively the first example I gave, but without assignment to a variable.