Python - for loop - understanding

Question

I had a problem with a for loop in a python script. The problem is meanwhile solved, but I do not understand the necessity of the comma that solved the issue.

this was the faulty for loop:

variable= (["abc.com", ["", "test"]])
for a,b in variable:
print(a,b)

result:

Traceback (most recent call last):
File "", line 1, in
ValueError: too many values to unpack (expected 2)

this repaired the faulty for-loop:

variable= (["abc.com", ["", "test"]],)
for a,b in variable:
print(a,b)

result:

abc.com ['', 'test']

Why is this comma necessary before the closing bracket? If I extend the content inside the variable, there is no comma necessary at the end.

without comma at the end:

variable= (["abc.com", ["", "test"]], ["xyz.com", ["", "test2"]])
for a,b in variable:
print(a,b)

result:

abc.com ['', 'test']
xyz.com ['', 'test2']

with comma at the end:

variable= (["abc.com", ["", "test"]], ["xyz.com", ["", "test2"]],)
for a,b in variable:
print(a,b)

result:

abc.com ['', 'test']
xyz.com ['', 'test2']

Any idea why there is sometimes the last comma necessary and sometimes not?

Thanks

Ture Pålsson · Accepted Answer · 2020-06-15 15:46:25Z

2

The assignment of variable in your first example is equivalent to

variable = ["abc.com", ["", "test"]]

i.e, the value will be a single list; the outer parentheses are redundant. When you loop over that, the first item is the string "abc.com", which won't match a, b — there are too many letters. By adding a comma after the list, you turn the expression into a tuple. If you have more than one element, there is already a comma there (after the first element), so you don't need to add another one.

The takeaway is: Parentheses don't make tuples; commas do! Consider the following assignments

x = 1   # Integer
x = (1) # Also integer
x = 1,  # One-element tuple
x = (1,) # Also one-element tuple
x = 1,2  # Two-element tuple

edited Jun 15, 2020 at 15:46

answered Jun 14, 2020 at 19:11

Ture Pålsson

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