Could not find a version that satisfies the requirement pytz

Question

I have a problem installing pytz in virtualenv.

Downloading/unpacking pytz
Could not find a version that satisfies the requirement pytz (from versions: 2009r, 2008b, 2009f, 2008c, 2007g, 2011g, 2005m, 2011e, 2007f, 2011k, 2007k, 2006j, 2008h, 2008i, 2011e, 2008a, 2009e, 2006g, 2011j, 2010l, 2005m, 2008i, 2005k, 2008g, 2007c, 2007i, 2009l, 2009r, 2006j, 2011k, 2007d, 2006p, 2009i, 2009u, 2007i, 2009f, 2010g, 2008h, 2009a, 2007g, 2011e, 2006p, 2012b, 2010k, 2005r, 2007f, 2009l, 2009p, 2008c, 2009j, 2008g, 2010g, 2010h, 2011h, 2010k, 2007c, 2007d, 2011d, 2009l, 2011c, 2008a, 2005m, 2007k, 2009n, 2011d, 2010o, 2013b, 2012h, 2010e, 2012c, 2012d, 2012f, 2011n, 2011b, 2011j, 2008c, 2012j, 2007k, 2009f, 2009d, 2010e, 2010b, 2013b, 2011d, 2009p, 2008h, 2005r, 2009i, 2009n, 2009a, 2010k, 2008g, 2006g, 2008b, 2012c, 2009i, 2007g, 2012c, 2010h, 2011n, 2012g, 2007d, 2008a, 2009u, 2012g, 2010o, 2006p, 2010b, 2009u, 2012d, 2011k, 2012f, 2009a, 2007f, 2011h, 2010l, 2009j, 2011g, 2009g, 2009g, 2005r, 2011c, 2012g, 2009g, 2012d, 2009j, 2010o, 2007c, 2010g, 2006g, 2009d, 2010h, 2005k, 2006j, 2010b, 2009n, 2011g, 2011c, 2012b, 2009e, 2009d, 2011j, 2007i, 2012j, 2010l, 2009r, 2012h, 2010e, 2009p, 2008i, 2012f, 2009e, 2012b, 2011h, 2005k, 2008b, 2013b, 2011n, 2012j, 2004b)
Cleaning up...
No distributions matching the version for pytz

It seems like it's a problem with the latest version of pip.

Is there any workaround?

Solution

This error occurs when installing pytz using pip v1.4 or newer, due to this change in behaviour:

Pre-release Versions

Starting with v1.4, pip will only install stable versions as specified by PEP426 by default. If a version cannot be parsed as a compliant PEP426 version then it is assumed to be a pre-release.

If a Requirement specifier includes a pre-release or development version (e.g. >=0.0.dev0) then pip will allow pre-release and development versions for that requirement. This does not include the != flag.

The pip install command also supports a --pre flag that will enable installing pre-releases and development releases.

The version identifiers for the pytz package have a format like 2013b. PEP426 uses the version identifiers described in PEP440, which specifies that:

Public version identifiers MUST comply with the following scheme:

N[.N]+[{a|b|c|rc}N][.postN][.devN]

Because the pytz versions like 2013b do not match this format, version 1.4+ of pip is treating all versions of pytz as pre-release versions, and is not installing them by default.

If you are only installing pytz, you can use the --pre flag to avoid this behaviour, but you wouldn't want to use this flag for installing your entire project's requirements: some packages might have unstable pre-release versions you don't want. In that case, use the behaviour described above: if you specify a "pre-release" version number for the package, then pip will search for "pre-release" versions of the package. So I've added this to my requirements.txt:

pytz>=2013b

When I upgrade my packages, pip will now correctly search for and install the latest version of pytz.

This has been filed as issue #1204837 in the pytz bug tracker and issue #974 in the pip bug tracker.

Stop Press: As described in the PyTz bug report, the version numbering of pytz has now been changed to, for example, 2013.7 - so once you have upgraded to this, the problem should no longer occur.