pip is capable of determining and installing the dependencies of packages. Theprocess of determining which version of a dependency to install is known asdependency 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 workout 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 dependencyinformation of the requested packages. It needs to work out the dependenciesof the requested packages, the dependencies of those dependencies, and so on.Over the course of the dependency resolution process, pip will need to downloaddistribution files of the packages which are used to get the dependencies of apackage.
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 packageversions it needs to install and, later, check these assumptions were notincorrect. When pip finds that an assumption it made earlier is incorrect, ithas to backtrack, which means also discarding some of the work that has alreadybeen 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 ofchoices made during is not unexpected behaviour or a bug. It is part of howdependency resolution for Python packages works.
Example
The user requests pip install tea. The package tea declares a dependency onhot-water, spoon, cup, amongst others.
pip starts by picking the most recent version of tea and get the list ofdependencies of that version of tea. It will then repeat the process forthose 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 theversion of spoon it has chosen. Thus, pip will “go back” (backtrack) and tryto use another version of cup. If it is successful, it will continue onto thenext package (like sugar). Otherwise, it will continue to backtrack on cupuntil it finds a version of cup that is compatible with all the otherpackages.
This can look like:
$ pip install teaCollecting tea Downloading tea-1.9.8-py2.py3-none-any.whl (346 kB) |████████████████████████████████| 346 kB 10.4 MB/sCollecting spoon==2.27.0 Downloading spoon-2.27.0-py2.py3-none-any.whl (312 kB) |████████████████████████████████| 312 kB 19.2 MB/sCollecting cup>=1.6.0 Downloading cup-3.22.0-py2.py3-none-any.whl (397 kB) |████████████████████████████████| 397 kB 28.2 MB/sINFO: pip is looking at multiple versions of this package to determinewhich 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/sINFO: pip is looking at multiple versions of this package to determinewhich 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 backtrackingchoices it is making during dependency resolution.
If pip starts backtracking during dependency resolution, it does not know howmany 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 startsbacktracking. In the case where a package has a lot of versions, arriving at agood candidate can take a lot of time. The amount of time depends on thepackage size, the number of versions pip must try, and various other factors.
Backtracking reduces the risk that installing a new package will accidentallybreak an existing installed package, and so reduces the risk that yourenvironment gets messed up. To do this, pip has to do more work, to find outwhich 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 backtrackingexcessively during dependency resolution. There are ways to reduce thedegree to which pip might backtrack though. Nearly all of these approachesrequire 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 yourpreferred option.
However, it is a possible that pip will not be able to find a set ofcompatible versions. For this, pip will try every possible combination thatit 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 thestrategies 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:
$ python -m pip install tea "cup >= 3.13"
$ python -m pip install tea "cup >= 3.13"
C:> py -m pip install tea "cup >= 3.13"
This will reduce the number of versions of cup it tries, andpossibly reduce the time pip takes to install.
There is a possibility that the addition constraint is incorrect. When thishappens, the reduced search space makes it easier for pip to more quicklydetermine what caused the conflict and present that to the user. It could alsoresult 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 knowhow 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 andversion number for each dependency of that package. You can create thiswith pip-tools.
This means the “work” is done once during development process, and thuswill avoid performing dependency resolution during deployment.
Dealing with dependency conflicts#
This section provides practical suggestions to pip users who encountera ResolutionImpossible error, where pip cannot install their specifiedpackages due to conflicting dependencies.
Understanding your error message#
When you get a ResolutionImpossible error, you might see somethinglike this:
$ 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
$ 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
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 areasking for conflicting versions of pytest.
pytest-covversion2.12.1, requirespytestwith a version or equal to4.6.package_teaversion4.3.0depends on version2.3.1ofpackage_water
Sometimes these messages are straightforward to read, because they usecommonly understood comparison operators to specify the required version(e.g. < or >).
However, Python packaging also supports some more complex ways forspecifying package versions (e.g. ~= or *):
Operator | Description | Example |
|---|---|---|
| Any version greater than the specified version. |
|
| Any version less than the specified version. |
|
| Any version less than or equal to the specified version. |
|
| Any version greater than or equal to the specified version. |
|
| Exactly the specified version. |
|
| Any version not equal to the specified version. |
|
| Any compatible1 version. |
|
| Can be used at the end of a version number to represent all. |
|
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 befound in PEP 440.
Possible solutions#
The solution to your error will depend on your individual use case. Hereare some things to try:
Audit your top level requirements#
As a first step, it is useful to audit your project and remove anyunnecessary or out of date requirements (e.g. from your setup.py orrequirements.txt files). Removing these can significantly reduce thecomplexity of your dependency tree, thereby reducing opportunities forconflicts to occur.
Loosen your top level requirements#
Sometimes the packages that you have asked pip to install areincompatible because you have been too strict when you specified thepackage version.
In our first example both package_coffee and package_tea have beenpinned 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 onthe 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_coffeeandpackage_teaby removing the version specifiers altogether (e.g.pip install package_coffee package_tea)
In the second case, pip will automatically find a version of bothpackage_coffee and package_tea that depend on the same version ofpackage_water, installing:
package_coffee 0.46.0b0, which depends onpackage_water 2.6.1package_tea 4.3.0which also depends onpackage_water 2.6.1
If you want to prioritize one package over another, you can add versionspecifiers to only the more important package:
$ python -m pip install package_coffee==0.44.1b0 package_tea
$ python -m pip install package_coffee==0.44.1b0 package_tea
C:> py -m pip install package_coffee==0.44.1b0 package_teaThis will result in:
package_coffee 0.44.1b0, which depends onpackage_water 2.6.1package_tea 4.1.3which also depends onpackage_water 2.6.1
Now that you have resolved the issue, you can repin the compatiblepackage versions as required.
Loosen the requirements of your dependencies#
Assuming that you cannot resolve the conflict by loosening the versionof the package you require (as above), you can try to fix the issue onyour 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 ofany 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 packageversions 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 (forexample, 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 askfor help on:
See “How do I ask a good question?” for tips on asking for help.
Unfortunately, the pip team cannot provide support for individualdependency conflict errors. Please only open a ticket onpip’s issue tracker if you believethat your problem has exposed a bug in pip.
