Could not find a version that satisfies the requirement pytz

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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.

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