Reorganise documentation related to dependency resolution

docs/html/topics/dependency-resolution.md

@@ -106,3 +106,212 @@ Backtracking reduces the risk that installing a new package will accidentally
 break an existing installed package, and so reduces the risk that your
 environment gets messed up. To do this, pip has to do more work, to find out
 which version of a package is a good candidate to install.
+
+## Possible ways to reduce backtracking
+
+There is no one-size-fits-all answer to situations where pip is backtracking
+excessively during dependency resolution. There are ways to reduce the
+degree to which pip might backtrack though. Nearly all of these approaches
+require some amount of trial and error.
+
+### Allow pip to complete its backtracking
+
+In most cases, pip will complete the backtracking process successfully.
+This could take a very long time to complete, so this may not be your
+preferred option.
+
+However, it is a possible that pip will not be able to find a set of
+compatible versions. For this, pip will try every possible combination that
+it needs to and determine that there is no compatible set.
+
+If you'd prefer not to wait, you can interrupt pip (Ctrl+c) and try the
+strategies listed below.
+
+### Reduce the number of versions pip is trying to use
+
+It is usually a good idea to add constraints the package(s) that pip is backtracking on (e.g. in the above example - `cup`).
+
+You could try something like:
+
+```{pip-cli}
+$ pip install tea "cup >= 3.13"
+```
+
+This will reduce the number of versions of `cup` it tries, and
+possibly reduce the time pip takes to install.
+
+There is a possibility that the addition constraint is incorrect. When this
+happens, the reduced search space makes it easier for pip to more quickly
+determine what caused the conflict and present that to the user. It could also
+result in pip backtracking on a different package due to some other conflict.
+
+### Use constraint files or lockfiles
+
+This option is a progression of the previous section. It requires users to know
+how to inspect:
+
+- the packages they're trying to install
+- the package release frequency and compatibility policies
+- their release notes and changelogs from past versions
+
+During deployment, you can create a lockfile stating the exact package and
+version number for for each dependency of that package. You can create this
+with `pip-tools <https://github.com/jazzband/pip-tools/>`\_\_.
+
+This means the "work" is done once during development process, and thus
+will avoid performing dependency resolution during deployment.
+
+## Dealing with dependency conflicts
+
+This section provides practical suggestions to pip users who encounter
+a `ResolutionImpossible` error, where pip cannot install their specified
+packages due to conflicting dependencies.
+
+### Understanding your error message
+
+When you get a `ResolutionImpossible` error, you might see something
+like this:
+
+```{pip-cli}
+$ pip install "pytest < 4.6" pytest-cov==2.12.1
+[regular pip output]
+ERROR: Cannot install pytest-cov==2.12.1 and pytest<4.6 because these package versions have conflicting dependencies.
+
+The conflict is caused by:
+    The user requested pytest<4.6
+    pytest-cov 2.12.1 depends on pytest>=4.6
+```
+
+In this example, pip cannot install the packages requested because they are
+asking for conflicting versions of pytest.
+
+- `pytest-cov` version `2.12.1`, requires `pytest` with a version or equal to
+  `4.6`.
+- `package_tea` version `4.3.0` depends on version `2.3.1` of
+  `package_water`
+
+Sometimes these messages are straightforward to read, because they use
+commonly understood comparison operators to specify the required version
+(e.g. `<` or `>`).
+
+However, Python packaging also supports some more complex ways for
+specifying package versions (e.g. `~=` or `*`):
+
+| Operator | Description                                                    | Example                                             |
+| -------- | -------------------------------------------------------------- | --------------------------------------------------- |
+| `>`      | Any version greater than the specified version.                | `>3.1`: any version greater than `3.1`.             |
+| `<`      | Any version less than the specified version.                   | `<3.1`: any version less than `3.1`.                |
+| `<=`     | Any version less than or equal to the specified version.       | `<=3.1`: any version less than or equal to `3.1`.   |
+| `>=`     | Any version greater than or equal to the specified version.    | `>=3.1`: version `3.1` and greater.                 |
+| `==`     | Exactly the specified version.                                 | `==3.1`: only `3.1`.                                |
+| `!=`     | Any version not equal to the specified version.                | `!=3.1`: any version other than `3.1`.              |
+| `~=`     | Any compatible{sup}`1` version.                                | `~=3.1`: any version compatible{sup}`1` with `3.1`. |
+| `*`      | Can be used at the end of a version number to represent _all_. | `==3.1.*`: any version that starts with `3.1`.      |
+
+{sup}`1` Compatible versions are higher versions that only differ in the final segment.
+`~=3.1.2` is equivalent to `>=3.1.2, ==3.1.*`. `~=3.1` is equivalent to `>=3.1, ==3.*`.
+
+The detailed specification of supported comparison operators can be
+found in {pep}`440`.
+
+### Possible solutions
+
+The solution to your error will depend on your individual use case. Here
+are some things to try:
+
+#### Audit your top level requirements
+
+As a first step, it is useful to audit your project and remove any
+unnecessary or out of date requirements (e.g. from your `setup.py` or
+`requirements.txt` files). Removing these can significantly reduce the
+complexity of your dependency tree, thereby reducing opportunities for
+conflicts to occur.
+
+#### Loosen your top level requirements
+
+Sometimes the packages that you have asked pip to install are
+incompatible because you have been too strict when you specified the
+package version.
+
+In our first example both `package_coffee` and `package_tea` have been
+_pinned_ to use specific versions
+(`package_coffee==0.44.1b0 package_tea==4.3.0`).
+
+To find a version of both `package_coffee` and `package_tea` that depend on
+the same version of `package_water`, you might consider:
+
+- Loosening the range of packages that you are prepared to install
+  (e.g. `pip install "package_coffee>0.44.*" "package_tea>4.0.0"`)
+- Asking pip to install _any_ version of `package_coffee` and `package_tea`
+  by removing the version specifiers altogether (e.g.
+  `pip install package_coffee package_tea`)
+
+In the second case, pip will automatically find a version of both
+`package_coffee` and `package_tea` that depend on the same version of
+`package_water`, installing:
+
+- `package_coffee 0.46.0b0`, which depends on `package_water 2.6.1`
+- `package_tea 4.3.0` which _also_ depends on `package_water 2.6.1`
+
+If you want to prioritize one package over another, you can add version
+specifiers to _only_ the more important package:
+
+```{pip-cli}
+$ pip install package_coffee==0.44.1b0 package_tea
+```
+
+This will result in:
+
+- `package_coffee 0.44.1b0`, which depends on `package_water 2.6.1`
+- `package_tea 4.1.3` which also depends on `package_water 2.6.1`
+
+Now that you have resolved the issue, you can repin the compatible
+package versions as required.
+
+#### Loosen the requirements of your dependencies
+
+Assuming that you cannot resolve the conflict by loosening the version
+of the package you require (as above), you can try to fix the issue on
+your _dependency_ by:
+
+- Requesting that the package maintainers loosen _their_ dependencies
+- Forking the package and loosening the dependencies yourself
+
+:::{warning}
+If you choose to fork the package yourself, you are _opting out_ of
+any support provided by the package maintainers. Proceed at your own risk!
+:::
+
+#### All requirements are appropriate, but a solution does not exist
+
+Sometimes it's simply impossible to find a combination of package
+versions that do not conflict. Welcome to [dependency hell].
+
+In this situation, you could consider:
+
+- Using an alternative package, if that is acceptable for your project.
+  See [Awesome Python] for similar packages.
+- Refactoring your project to reduce the number of dependencies (for
+  example, by breaking up a monolithic code base into smaller pieces).
+
+### Getting help
+
+If none of the suggestions above work for you, we recommend that you ask
+for help on:
+
+- [Python user Discourse](https://discuss.python.org/c/users/7)
+- [Python user forums](https://www.python.org/community/forums/)
+- [Python developers Slack channel](https://pythondev.slack.com/)
+- [Python IRC](https://www.python.org/community/irc/)
+- [Stack Overflow](https://stackoverflow.com/questions/tagged/python)
+
+See ["How do I ask a good question?"] for tips on asking for help.
+
+Unfortunately, **the pip team cannot provide support for individual
+dependency conflict errors**. Please _only_ open a ticket on
+[pip's issue tracker](https://github.com/pypa/pip/issues) if you believe
+that your problem has exposed a bug in pip.
+
+["how do i ask a good question?"]: https://stackoverflow.com/help/how-to-ask
+[awesome python]: https://python.libhunt.com/
+[dependency hell]: https://en.wikipedia.org/wiki/Dependency_hell