# 2.4 Relationships between Distributions and Packages

A distribution may relate to packages in three specific ways:

1. It can require packages or modules.

2. It can provide packages or modules.

3. It can obsolete packages or modules.

These relationships can be specified using keyword arguments to the `distutils.core.setup()` function.

Dependencies on other Python modules and packages can be specified by supplying the requires keyword argument to `setup()`. The value must be a list of strings. Each string specifies a package that is required, and optionally what versions are sufficient.

To specify that any version of a module or package is required, the string should consist entirely of the module or package name. Examples include `'mymodule'` and `'xml.parsers.expat'`.

If specific versions are required, a sequence of qualifiers can be supplied in parentheses. Each qualifier may consist of a comparison operator and a version number. The accepted comparison operators are:

    <    >    ==
    <=   >=   !=

These can be combined by using multiple qualifiers separated by commas (and optional whitespace). In this case, all of the qualifiers must be matched; a logical AND is used to combine the evaluations.

Let's look at a bunch of examples:

Requires Expression | Explanation
--- | ---
`==1.0` | Only version `1.0` is compatible
`>1.0, !=1.5.1, <2.0` | Any version after `1.0` and before `2.0` is compatible, except `1.5.1`

Now that we can specify dependencies, we also need to be able to specify what we provide that other distributions can require. This is done using the provides keyword argument to `setup()`. The value for this keyword is a list of strings, each of which names a Python module or package, and optionally identifies the version. If the version is not specified, it is assumed to match that of the distribution.

Some examples:

Provides Expression | Explanation
--- | ---
`mypkg` | Provide `mypkg`, using the distribution version
`mypkg (1.1)` | Provide `mypkg` version 1.1, regardless of the distribution version

A package can declare that it obsoletes other packages using the obsoletes keyword argument. The value for this is similar to that of the requires keyword: a list of strings giving module or package specifiers. Each specifier consists of a module or package name optionally followed by one or more version qualifiers. Version qualifiers are given in parentheses after the module or package name.

The versions identified by the qualifiers are those that are obsoleted by the distribution being described. If no qualifiers are given, all versions of the named module or package are understood to be obsoleted.