Dependency Resolution¶

pip is capable of determining and installing the dependencies of packages. The process of determining which version of a dependency to install is known as dependency resolution. This behaviour can be disabled by passing --no-deps to pip install.

How it works¶

When a user does a pip install (e.g. pip install tea), pip needs to work out the package’s dependencies (e.g. spoon, hot-water, tea-leaves etc.) and what the versions of each of those dependencies it should install.

At the start of a pip install run, pip does not have all the dependency information of the requested packages. It needs to work out the dependencies of the requested packages, the dependencies of those dependencies, and so on. Over the course of the dependency resolution process, pip will need to download distribution files of the packages which are used to get the dependencies of a package.

Backtracking¶

Changed in version 20.3: pip’s dependency resolver is now capable of backtracking.

During dependency resolution, pip needs to make assumptions about the package versions it needs to install and, later, check these assumptions were not incorrect. When pip finds that an assumption it made earlier is incorrect, it has to backtrack, which means also discarding some of the work that has already been done, and going back to choose another path.

This can look like pip downloading multiple versions of the same package, since pip explicitly presents each download to the user. The backtracking of choices made during is not unexpected behaviour or a bug. It is part of how dependency resolution for Python packages works.

Example

The user requests pip install tea. The package tea declares a dependency on hot-water, spoon, cup, amongst others.

pip starts by picking the most recent version of tea and get the list of dependencies of that version of tea. It will then repeat the process for those packages, picking the most recent version of spoon and then cup. Now, pip notices that the version of cup it has chosen is not compatible with the version of spoon it has chosen. Thus, pip will “go back” (backtrack) and try to use another version of cup. If it is successful, it will continue onto the next package (like sugar). Otherwise, it will continue to backtrack on cup until it finds a version of cup that is compatible with all the other packages.

This can look like:

$ pip install tea
Collecting tea
  Downloading tea-1.9.8-py2.py3-none-any.whl (346 kB)
     |████████████████████████████████| 346 kB 10.4 MB/s
Collecting spoon==2.27.0
  Downloading spoon-2.27.0-py2.py3-none-any.whl (312 kB)
     |████████████████████████████████| 312 kB 19.2 MB/s
Collecting cup>=1.6.0
  Downloading cup-3.22.0-py2.py3-none-any.whl (397 kB)
     |████████████████████████████████| 397 kB 28.2 MB/s
INFO: pip is looking at multiple versions of this package to determine
which version is compatible with other requirements.
This could take a while.
  Downloading cup-3.21.0-py2.py3-none-any.whl (395 kB)
     |████████████████████████████████| 395 kB 27.0 MB/s
  Downloading cup-3.20.0-py2.py3-none-any.whl (394 kB)
     |████████████████████████████████| 394 kB 24.4 MB/s
  Downloading cup-3.19.1-py2.py3-none-any.whl (394 kB)
     |████████████████████████████████| 394 kB 21.3 MB/s
  Downloading cup-3.19.0-py2.py3-none-any.whl (394 kB)
     |████████████████████████████████| 394 kB 26.2 MB/s
  Downloading cup-3.18.0-py2.py3-none-any.whl (393 kB)
     |████████████████████████████████| 393 kB 22.1 MB/s
  Downloading cup-3.17.0-py2.py3-none-any.whl (382 kB)
     |████████████████████████████████| 382 kB 23.8 MB/s
  Downloading cup-3.16.0-py2.py3-none-any.whl (376 kB)
     |████████████████████████████████| 376 kB 27.5 MB/s
  Downloading cup-3.15.1-py2.py3-none-any.whl (385 kB)
     |████████████████████████████████| 385 kB 30.4 MB/s
INFO: pip is looking at multiple versions of this package to determine
which version is compatible with other requirements.
This could take a while.
  Downloading cup-3.15.0-py2.py3-none-any.whl (378 kB)
     |████████████████████████████████| 378 kB 21.4 MB/s
  Downloading cup-3.14.0-py2.py3-none-any.whl (372 kB)
     |████████████████████████████████| 372 kB 21.1 MB/s

These multiple Downloading cup-{version} lines show that pip is backtracking choices it is making during dependency resolution.

If pip starts backtracking during dependency resolution, it does not know how many choices it will reconsider, and how much computation would be needed.

For the user, this means it can take a long time to complete when pip starts backtracking. In the case where a package has a lot of versions, arriving at a good candidate can take a lot of time. The amount of time depends on the package size, the number of versions pip must try, and various other factors.

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:

Linux

$ python -m pip install tea "cup >= 3.13"

MacOS

$ python -m pip install tea "cup >= 3.13"

Windows

C:> py -m 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 each dependency of that package. You can create this with 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:

Linux

$ python -m 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

MacOS

$ python -m 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

Windows

C:> py -m 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 compatible¹ version.	`~=3.1`: any version compatible¹ 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`.

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:

Linux

$ python -m pip install package_coffee==0.44.1b0 package_tea

MacOS

$ python -m pip install package_coffee==0.44.1b0 package_tea

Windows

C:> py -m 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:

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 if you believe that your problem has exposed a bug in pip.