Varformat Library

Varformat can format and un-format (parse) strings containing various styles of variables.

>>> import varformat as vf
>>> vf.format('Hi ${name}!', name='mom')
'Hi mom!'
>>> vf.parse('archive-${date}.tar.gz', 'archive-1970-01-01.tar.gz')
{'date': '1970-01-01'}

>>> from varformat.formats import python
>>> python.format('Classic {style}', style='python braces')
'Classic python braces'

>>> from varformat.formats import posix_shell as sh
>>> sh.format('POSIX compliant $style', style='dollar variables')
'POSIX compliant dollar variables'

Getting Started

Varformat is available to install via pip:

pip install varformat

When installed, the modules varformat and varformat.formats will be available. Global functions format, vformat, and parse represent the default formmatter with a ${} style:

>>> import varformat as vf
>>> vf.format('my name ${name}', name='jeff')
'my name jeff'

If it is necessary to specify keys which are not valid python identifiers, such as numbers or string with spaces, you can use vformat instead:

>>> import varformat as vf
>>> vf.vformat('My three favorite foods: ${1}, ${2}, and ${1} again',
...     {'1': 'pizza', '2': 'chocolate'})
'My three favorite foods: pizza, chocolate, and pizza again'

vformat also supports keyword arguments to customize formatting behavior. partial_ok (default False) and extra_ok (default: True) control whether it is allowed to provide less (or more) arguments than the format string requires. ambiguity_check (default: False) will raise an error if your resulting string will be ambiguous:

>>> import varformat as vf
>>> vf.vformat('package-${os}-${arch}', {'os': 'ubuntu-22.04', 'arch': 'amd64'}, ambiguity_check=True)
Traceback (most recent call last):
    ...
varformat.AmbiguityError: refusing to format because parsing would be ambiguous:
  could be: {'os': 'ubuntu-22.04', 'arch': 'amd64'}
        or: {'os': 'ubuntu', 'arch': '22.04-amd64'}

The parse function, which performs the inverse of vformat, also supports ambiguity_check (default: True):

>>> import varformat as vf
>>> vf.parse('package-${os}-${arch}', 'package-ubuntu-22.04-amd64')
Traceback (most recent call last):
    ...
varformat.AmbiguityError: parsing is ambiguous:
  could be: {'os': 'ubuntu-22.04', 'arch': 'amd64'}
        or: {'os': 'ubuntu', 'arch': '22.04-amd64'}

You can of course set ambiguity_check to False, and parse will parse using the regular expression rules (greedily).

Other formatters

Module varformat.formats contains formatters with other syntaxes:

• varformat.formats.posix_shell follows POSIX shell variable rules: it disallows numeric identifiers, identifiers with spaces, but allows referencing variables like $var in addition to ${var};
• varformat.formats.python follows classic python format string rules (e.g. {var}).

You can define your own formatter with your own custom syntax by subclassing either varformat.RegexFormatter and defining a regular expression that detects placeholders, or varformat.AbstractFormatter and defining a parsing function. See class docstrings for more information.