I'm somewhat surprised when I have code like `"hello %s".format(person)` in my project there is no exception raised and "hello %s" returned. Have several tries it seems `str.format` won't check argument number.

>>> '{} {}'.format(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: tuple index out of range
>>> '{} {}'.format(1, 2, 3)
'1 2'
>>> ''.format(1, 2, 3)
''
>>>

The IndexError is not ideal. I think this behavior could be improved since it's highly possible such code is an oversight bug. The old format string does a good job in this place:

>>> "%s %s" % 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: not enough arguments for format string
>>> "%s %s" % (1,2,3)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: not all arguments converted during string formatting
>>> "" % 1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: not all arguments converted during string formatting

This will break the following case:

def geterrmsg(n):
    return ('function doesn't have arguments' if n == 0 else
            'function have a single argument' if n == 1 else
            'function have %d arguments')

print(geterrmsg(n).format(n))

Actually geterrmsg() can take the error message from a translations database, and the number of different cases can be dependent on the language.

Yes, this is a breaking change. A currently working piece of code suddenly raises an exception.

But, please note Serhiy in your example you have the same bug as me, you've gotten an oversight bug in it: it's {:d} not %s. That's why I want the improvement.

This was a deliberate design choice, and as Serhiy notes, at least partially driven by translations. It seems to me it would be a job for a linter to point out a problem, if the string is a constant.

If issue28308 be implemented, it may be that the Python compiler itself will emit a warning.

I still think it's a job for a linter, even if the compiler optimizes away the call to .format().

After reading PEP3101 and the archived mails, I think it's by design. Even if not, it might be too late to change.

I don't prefer either way, emitting warnings by compiler or linter. They can't give hints for runtime retrieved patterns, and not everyone turns on warning and uses linters.

I prefer using a customized string.Formatter, implementing my own `check_unused_args`, raising appropriate exceptions, and used it across my project instead of the default `str.format`. But there seems some problems with string.Formatter.

stage: resolved