IndexError: tuple index out of range when parsing method arguments

Question

I've already checked this question, but couldn't find an answer there. Here is a simple example that demonstrates my use case:

def log(*args):
    message = str(args[0])
    arguments = tuple(args[1:])
    # message itself
    print(message)
    # arguments for str.format()0
    print(arguments)
    # shows that arguments have correct indexes
    for index, value in enumerate(arguments):
        print("{}: {}".format(index, value))
    # and amount of placeholders == amount of arguments
    print("Amount of placeholders: {}, Amount of variables: {}".format(message.count('{}'), len(arguments)))

    # But this still fails! Why?
    print(message.format(arguments))

log("First: {}, Second: {}, Third: {}, Fourth: {}", "asdasd", "ddsdd", "12312333", "fdfdf")

And the output:

First: {}, Second: {}, Third: {}, Fourth: {}
('asdasd', 'ddsdd', '12312333', 'fdfdf')
0: asdasd
1: ddsdd
2: 12312333
3: fdfdf
Amount of placeholders: 4, Amount of variables: 4
Traceback (most recent call last):
  File "C:/Users/sbt-anikeev-ae/IdeaProjects/test-this-thing-on-python/test-this-thing.py", line 12, in <module>
    log("First: {}, Second: {}, Third: {}, Fourth: {}", "asdasd", "ddsdd", "12312333", "fdfdf")
  File "C:/Users/sbt-anikeev-ae/IdeaProjects/test-this-thing-on-python/test-this-thing.py", line 10, in log
    print(message.format(arguments))
IndexError: tuple index out of range

P.S: I've already refused using such a method (that wraps str.format()), as it seems to be excess. But still it puzzles me, why wouldn't this work as expected?

Jean-François Fabre · Accepted Answer · 2017-01-13 10:09:14Z

11

you have to use * to unpack the tuple into actual arguments for format:

print(message.format(*arguments))

otherwise, arguments is seen as the sole argument of format (and it works for the first {} occurrence, by converting your tuple to string, but chokes when it encounters the second occurrence of {})

answered Jan 13, 2017 at 10:09

Jean-François Fabre♦

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Community · Accepted Answer · 2017-05-23 11:47:35Z

1

You need to pass arguments not tuple. This is done by using '*arguments'. Expanding tuples into arguments

def log(*args):
    message = str(args[0])
    arguments = tuple(args[1:])
    # message itself
    print(message)
    # arguments for str.format()0
    print(arguments)
    # shows that arguments have correct indexes
    for index, value in enumerate(arguments):
        print("{}: {}".format(index, value))
    # and amount of placeholders == amount of arguments
    print("Amount of placeholders: {}, Amount of variables: {}".format(message.count('{}'), len(arguments)))

    # But this still fails! Why?
    print(type(arguments))
    print(message.format(*arguments))

log("First: {}, Second: {}, Third: {}, Fourth: {}", "asdasd", "ddsdd", "12312333", "fdfdf")

edited May 23, 2017 at 11:47

CommunityBot

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answered Jan 13, 2017 at 10:12

ppasler

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Comments

user650881 · Accepted Answer · 2017-01-13 10:24:31Z

0

Try this

print(message.format(*arguments))

format does not expect a tuple

edited Jan 13, 2017 at 10:24

answered Jan 13, 2017 at 10:14

user650881

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