Python library for string formatting with custom placeholders

Question

I am searching for a Python library for string formatting with custom placeholders. What I mean is that I would like to be able to define a string like "%f %d %3c" where for example %f would be then replaced with some filename, %d with a directory name and %3c with a counter with three digits. Or something. It does not have to be printf-like but it would be great if it would be. So I would like to be able to define what each letter means, is it string or number, and also some formatting (like number of digits) data.

The idea is that user can specify the format and I then fill it in with data. Like datefmt works. But for custom things.

Is there something like that already made for Python (2.5+, sadly not for 2.7 and 3 and its __format__)?

utdemir · Accepted Answer · 2011-08-13 12:33:17Z

2

There is string.Template, but it doesn't provide exactly what you want:

>>> from string import Template
>>> t = Template("$filename $directory $counter")
>>> t.substitute(filename="file.py", directory="/home", counter=42)
'file.py /home 42'
>>> t.substitute(filename="file2.conf", directory="/etc", counter=8)
'file2.conf /etc 8'

Documentation: http://docs.python.org/library/string.html#template-strings

But I think this provides what you need. Just specify a template string and use this:

>>> template = "%(filename)s %(directory)s %(counter)03d"
>>> template % {"filename": "file", "directory": "dir", "counter": 42}
'file dir 042'
>>> template % {"filename": "file2", "directory": "dir2", "counter": 5}
'file2 dir2 005'

edited Aug 13, 2011 at 12:33

answered Aug 13, 2011 at 12:27

utdemir

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3 Comments

Mitar Over a year ago

Oh, I forgot about this. But it is safe/secure to let the user specify the formatting string? Or could this lead to some exploit?

Martin Thurau Over a year ago

As long as you don't put locals() as formatting arguments he will only have access to the dictionary you provide.

Matt Billenstein Over a year ago

I'd be careful about exposing the full formatting functionality -- think about things like: "%030000000000000d" % (3) which would require allocating a huge block of memory in advance. Probably tons of other possible vulnerabilities here...

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