Behavior of parenthesis in for loop variable declaration

Question

Does the usage of parentheses have any effect whatsoever in the variable declaration syntax of a Python for loop?

Example 1: basic for loop declaring i with no parenthesis

>>> for i in range(0,10): print(i)
...
0
1
2
3
4
5
6
7
8
9

Basic for loop declaring i with arbitrary parentheses

for (((((i))))) in range(0,10): print(i)
...
0
1
2
3
4
5
6
7
8
9

For loop which unpacks two values, declared with no parentheses

>>> for x,y in zip(range(0,10), range(0,10)):  print(x,y)
...
0 0
1 1
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9 9

Same thing, but with parenthesis around x, y, and both.

>>> for ((x),(y)) in zip(range(0,10), range(0,10)):  print(x,y)
...
0 0
1 1
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9 9

So it seems the parentheses have no effect - they are not interpreted as creating, for example, a tuple. Am I correct, or is there any reason to use parentheses in a for loop variable declaration?

You can even, apparently, say this:

>>> for [x,y] in zip(range(0,10), range(0,10)):  print(x,y)
...
0 0
1 1
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9 9

...and the list notation (brackets) appear to have no effect either.

No, there's not. Parentheses doesn't do anything in your code. If you want to construct a tuple, do it as (value,) — vaultah
– vaultah, Commented Jul 9, 2014 at 15:34

Martijn Pieters · Accepted Answer · 2014-07-09 15:36:40Z

5

Just parenthesis around an expression only act to group the expression (to override operator precedence, to be able to span multiple lines, etc.).

If you wanted to create tuples, use a comma:

>>> 1
1
>>> 1,
(1,)

answered Jul 9, 2014 at 15:36

Martijn Pieters

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Comments

score 2 · Accepted Answer · 2014-07-09 16:01:52Z

is there any reason to use parentheses in a for loop variable declaration?

Yes, you need them to unpack more complex iterables. Specifically, nested iterables such as this one:

enumerate(zip(range(10), range(10, 20)))

Using parenthesis, everything works fine:

>>> for x, (y, z) in enumerate(zip(range(10), range(10, 20))):
...    print("x=", x, "y=", y, "z=", z)
...
x= 0 y= 0 z= 10
x= 1 y= 1 z= 11
x= 2 y= 2 z= 12
x= 3 y= 3 z= 13
x= 4 y= 4 z= 14
x= 5 y= 5 z= 15
x= 6 y= 6 z= 16
x= 7 y= 7 z= 17
x= 8 y= 8 z= 18
x= 9 y= 9 z= 19
>>>

because x, (y, z) matches the structure of the iterables returned by:

enumerate(zip(range(10), range(10, 20)))

Without the parenthesis however, you will raise a ValueError:

>>> for x, y, z in enumerate(zip(range(10), range(10, 20))):
...    print("x=", x, "y=", y, "z=", z)
...
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: need more than 2 values to unpack
>>>

because Python sees x, y, z and therefore expects:

enumerate(zip(range(10), range(10, 20)))

to return three-item iterables when it only returns two-item ones (tuples with a number and another two-item tuple):

>>> for item in enumerate(zip(range(10), range(10, 20))):
...     print(item)
...
(0, (0, 10))
(1, (1, 11))
(2, (2, 12))
(3, (3, 13))
(4, (4, 14))
(5, (5, 15))
(6, (6, 16))
(7, (7, 17))
(8, (8, 18))
(9, (9, 19))
>>>

chepner · Accepted Answer · 2014-07-09 15:38:10Z

2

With the exception of the empty tuple (), it is not the parentheses which define a tuple, it's the commas. Parentheses simply keep the tuple items separate from surrounding code.

# These are all identical: x is a tuple with 2 elements.
x = (1, 2)
(x) = (1, 2)
x = 1, 2
(x) = 1, 2

# These are all identical: x is assigned the first element of the tuple (1, 2)
x, = (1, 2)
(x,) = (1, 2)
x, = 1, 2
(x,) = 1, 2

answered Jul 9, 2014 at 15:38

chepner

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