Write documentation for ResolutionImpossible error message

[nlhkabu]

Related to #8377

We need to write documentation to help users when they experience a ResolutionImpossible error message.

Archived drafts:

Version 1

Fixing conflicting dependencies

Note: the purpose of this documentation is to provide practical suggestions to pip users who encounter an error where pip cannot install their specified packages due to conflicting dependencies (a ResolutionImpossible error).
This documentation is not meant to act as a list of PyPA agreed best practices or recommendations, but rather an attempt to help users who find themselves stuck and are unsure what steps they can take to solve their problem.

Understanding your error message

When you get a ResolutionImpossible error, you might see something like this:

pip install ward==0.44.1b0 py2neo==4.3.0
Due to conflicting dependencies pip cannot install ward and py2neo:
- ward depends on pygments<3.0.0,>=2.4.2
- py2neo depends on pygments~=2.3.1

In this example, pip cannot install the packages you have requested, because they each depend on different versions of the same package (pygments):

• ward version 0.44.1b0 depends on a version of pygments that is less than 3.0.0 but greater than or equal to 2.4.2
• py2neo version 4.3.0 depends on a pygments release that is compatible with version 2.3.1

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. Here is the full list supported comparison operators:

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 version 3.1
!=	Any version not equal to the specified version	!=3.1: any version other than 3.1
~=	Any compatible release. Compatible releases are releases that are within the same major or minor version, assuming the package author is using semantic versioning.	~=3.1: version 3.1 or later, but not version 4.0 or later. ~=3.1.2: version 3.1.2 or later, but not version 3.2.0 or later.
*	Can be used at the end of a version number to represent "all"	== 3.1.*: any version that starts with 3.1. Equivalent to ~=3.1.0.

The full specification of supported comparison operators can be found in PEP440.

Error causes

There are three common causes for experiencing a ResolutionImpossible error.
The cause and possible solution to your error will depend on your individual use case.

1. Your "top level" requirements are too strict

In this scenario, the packages that you have asked pip to install are incompatible, possibly because you have been strict when you specified the package version.

In the example above both ward and py2neo have been pinned to use specific versions (ward==0.44.1b0 py2neo==4.3.0).

To find a version of both ward and py2neo that depend on the same version of pygments, you might consider:

• Loosening the range of packages that you are prepared to install (e.g. pip install "ward>0.44.*" "py2neo>4.0.0")
• Asking pip to install any version of ward and py2neo by removing the version specifiers altogether (e.g. pip install ward py2neo)

In the second case, pip will automatically find a version of both ward and py2neo that depend on the same version of pygments, installing:

• ward 0.46.0b0, which depends on pygments 2.6.1
• py2neo 4.3.0 which also depends on pygments 2.6.1

If you want to prioritise one package over another, you can add version specifiers to only the more important package:

pip install ward==0.44.1b0 py2neo

This will result in:

• ward 0.44.1b0, which depends on pygments 2.6.1
• py2neo 4.1.3 which also depends on pygments 2.6.1

Now that you have resolved the issue, you can repin the compatible package versions as required.

2. A package that you are using has requirements that are too strict

You may find yourself in a situation where a package you are trying to install has specific dependency requirements that create a conflict. This is known as a downstream dependency conflict.

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 the package itself by:

• Requesting that the package maintainers loosen their dependencies
• Forking the package and loosening the dependencies yourself

As a general rule, third party modules (libraries) should strive to reduce the the likelihood of causing dependency conflicts with other libraries by keeping their dependency requirements as loose as possible.

3. All requirements are loose, 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 can consider:

• Using an alternative package, if that is acceptable for your project (see Awesome Python for similar packages)
• Forking and refactoring one of your dependencies to avoid the conflict

Overriding dependency resolution (force installing)

There is currently no way to override pip's mechanism for resolving dependency conflicts and forcing pip to install an incompatible version of a required package.

If you would like to see this functionality please let the pip team know.

Version 2

Fixing conflicting dependencies

Note: the purpose of this documentation is to provide practical suggestions to pip users who encounter an error where pip cannot install their specified packages due to conflicting dependencies (a ResolutionImpossible error).
This documentation is not meant to act as a list of PyPA agreed best practices or recommendations, but rather an attempt to help users who find themselves stuck and are unsure what steps they can take to solve their problem.

Understanding your error message

When you get a ResolutionImpossible error, you might see something like this:

pip install packageA==0.44.1b0 packageB==4.3.0
Due to conflicting dependencies pip cannot install packageA and packageB:
- packageA depends on packageC<3.0.0,>=2.4.2
- packageB depends on packageC~=2.3.1

In this example, pip cannot install the packages you have requested, because they each depend on different versions of the same package (packageC):

• packageA version 0.44.1b0 depends on a version of packageC that is less than 3.0.0 but greater than or equal to 2.4.2
• packageB version 4.3.0 depends on a packageC release that is compatible with version 2.3.1

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. Here is the full list supported comparison operators:

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 version 3.1
!=	Any version not equal to the specified version	!=3.1: any version other than 3.1
~=	Any compatible release. Compatible releases are releases that are within the same major or minor version, assuming the package author is using semantic versioning.	~=3.1: version 3.1 or later, but not version 4.0 or later. ~=3.1.2: version 3.1.2 or later, but not version 3.2.0 or later.
*	Can be used at the end of a version number to represent "all"	== 3.1.*: any version that starts with 3.1. Equivalent to ~=3.1.0.

The full specification of supported comparison operators can be found in PEP440.

Error causes

There are three common causes for experiencing a ResolutionImpossible error.
The cause and possible solution to your error will depend on your individual use case.

1. Your "top level" requirements are too strict

In this scenario, the packages that you have asked pip to install are incompatible, possibly because you have been strict when you specified the package version.

In the example above both packageA and packageB have been pinned to use specific versions (packageA==0.44.1b0 packageB==4.3.0).

To find a version of both packageA and packageB that depend on the same version of packageC, you might consider:

• Loosening the range of packages that you are prepared to install (e.g. pip install "packageA>0.44.*" "packageB>4.0.0")
• Asking pip to install any version of packageA and packageB by removing the version specifiers altogether (e.g. pip install packageA packageB)

In the second case, pip will automatically find a version of both packageA and packageB that depend on the same version of packageC, installing:

• packageA 0.46.0b0, which depends on packageC 2.6.1
• packageB 4.3.0 which also depends on packageC 2.6.1

If you want to prioritise one package over another, you can add version specifiers to only the more important package:

pip install packageA==0.44.1b0 packageB

This will result in:

• packageA 0.44.1b0, which depends on packageC 2.6.1
• packageB 4.1.3 which also depends on packageC 2.6.1

Now that you have resolved the issue, you can repin the compatible package versions as required.

2. A package that you are using has requirements that are too strict

You may find yourself in a situation where a package you are trying to install has specific dependency requirements that create a conflict. This is known as a downstream dependency conflict.

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 the package itself by:

• Requesting that the package maintainers loosen their dependencies
• Forking the package and loosening the dependencies yourself

As a general rule, third party modules (libraries) should strive to reduce the the likelihood of causing dependency conflicts with other libraries by keeping their dependency requirements as loose as possible.

3. All requirements are loose, 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 can consider:

• Using an alternative package, if that is acceptable for your project (see Awesome Python for similar packages)
• Forking and refactoring one of your dependencies to avoid the conflict

Overriding dependency resolution (force installing)

There is currently no way to override pip's mechanism for resolving dependency conflicts and forcing pip to install an incompatible version of a required package.

If you would like to see this functionality please let the pip team know.

Version 3

Fixing conflicting dependencies

The purpose of this documentation is to provide practical suggestions to pip users who encounter an error where pip cannot install their specified packages due to conflicting dependencies (a ResolutionImpossible error).

Understanding your error message

When you get a ResolutionImpossible error, you might see something like this:

pip install packageA==0.44.1b0 packageB==4.3.0
Due to conflicting dependencies pip cannot install packageA and packageB:
- packageA depends on packageC<3.0.0,>=2.4.2
- packageB depends on packageC~=2.3.1

In this example, pip cannot install the packages you have requested, because they each depend on different versions of the same package (packageC):

• packageA version 0.44.1b0 depends on a version of packageC that is less than 3.0.0 but greater than or equal to 2.4.2
• packageB version 4.3.0 depends on a packageC release that is compatible with version 2.3.1

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. Here is the full list supported comparison operators:

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 version 3.1
!=	Any version not equal to the specified version	!=3.1: any version other than 3.1
~=	Any compatible release. Compatible releases are releases that are within the same major or minor version, assuming the package author is using semantic versioning.	~=3.1: version 3.1 or later, but not version 4.0 or later. ~=3.1.2: version 3.1.2 or later, but not version 3.2.0 or later.
*	Can be used at the end of a version number to represent "all"	== 3.1.*: any version that starts with 3.1. Equivalent to ~=3.1.0.

The full specification of supported comparison operators can be found in PEP440.

Error causes

There are three common causes for experiencing a ResolutionImpossible error.
The cause and possible solution to your error will depend on your individual use case.

1. Your "top level" requirements are too strict

In this scenario, the packages that you have asked pip to install are incompatible, possibly because you have been strict when you specified the package version.

In the example above both packageA and packageB have been pinned to use specific versions (packageA==0.44.1b0 packageB==4.3.0).

To find a version of both packageA and packageB that depend on the same version of packageC, you might consider:

• Loosening the range of packages that you are prepared to install (e.g. pip install "packageA>0.44.*" "packageB>4.0.0")
• Asking pip to install any version of packageA and packageB by removing the version specifiers altogether (e.g. pip install packageA packageB)

In the second case, pip will automatically find a version of both packageA and packageB that depend on the same version of packageC, installing:

• packageA 0.46.0b0, which depends on packageC 2.6.1
• packageB 4.3.0 which also depends on packageC 2.6.1

If you want to prioritise one package over another, you can add version specifiers to only the more important package:

pip install packageA==0.44.1b0 packageB

This will result in:

• packageA 0.44.1b0, which depends on packageC 2.6.1
• packageB 4.1.3 which also depends on packageC 2.6.1

Now that you have resolved the issue, you can repin the compatible package versions as required.

2. A package that you are using has requirements that are too strict

You may find yourself in a situation where a package you are trying to install has specific dependency requirements that create a conflict. This is known as a downstream dependency conflict.

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 the package itself by:

• Requesting that the package maintainers loosen their dependencies
• Forking the package and loosening the dependencies yourself

As a general rule, third party modules (libraries) should strive to reduce the the likelihood of causing dependency conflicts with other libraries by keeping their dependency requirements as loose as possible.

3. All requirements are loose, 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 can consider using an alternative package, if that is acceptable for your project. Seee Awesome Python for similar packages.

Getting help

If none of the suggestions above work for you, we recommend that you ask for help on:

• Python IRC
• Python user forums
• Python developers Slack channel
• Stack Overflow

See "How do I ask a good question?" for tips on asking for help.

Unfortunately, the pip team cannot provide support for individual conflict errors. Please only open a ticket on the pip issue tracker if you believe that your problem has exposed a bug in pip.

Version 4

Fixing conflicting dependencies (v. 17 June)

The purpose of this documentation is to provide practical suggestions to pip users who encounter an error where pip cannot install their specified packages due to conflicting dependencies (a ResolutionImpossible error).

Understanding your error message

When you get a ResolutionImpossible error, you might see something like this:

pip install packageA==0.44.1 packageB==4.3.0
Due to conflicting dependencies pip cannot install packageA and packageB:
- packageA depends on packageC<3.0.0,>=2.4.2
- packageB depends on packageC==2.3.1

In this example, pip cannot install the packages you have requested, because they each depend on different versions of the same package (packageC):

• packageA version 0.44.1 depends on a version of packageC that is less than 3.0.0 but greater than or equal to 2.4.2
• packageB version 4.3.0 depends on version 2.3.1 of packageC

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 *).

View the full list supported comparison operators

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 version 3.1
!=	Any version not equal to the specified version	!=3.1: any version other than 3.1
~=	Any compatible release. Compatible releases are releases that are within the same major or minor version, assuming the package author is using semantic versioning.	~=3.1: version 3.1 or later, but not version 4.0 or later. ~=3.1.2: version 3.1.2 or later, but not version 3.2.0 or later.
*	Can be used at the end of a version number to represent "all"	== 3.1.*: any version that starts with 3.1. Equivalent to ~=3.1.0.

The detailed specification of supported comparison operators can be found in PEP440.

Determining the origin of your conflict

In order to resolve your conflict, it's useful to understand that conflicts can occur between your "top level requirements" - packages you told pip to install - and "downstream dependencies" - packages required by your top level requirements.

1. Conflicts in your downstream dependencies

As in our example above, you may experience conflicts between your downstream dependencies (also known as "transitive dependencies"). This can happen at a high level:

Or deeper within your dependency tree:

2. Conflicts between your top level requirements and your downstream dependencies

Sometimes, a top level requirement is also a downstream dependency of one of your other packages:

To trace the origin of your conflict, you can run pip in verbose mode

Possible solutions

The solution to your error will depend on your individual use case. Here are some things to try:

1. Audit your top level requirements

As a first step it is useful to audit your project and remove any unneccessary or out of date requirements. Removing these can significantly reduce the complexity of your dependency tree, thereby reducing opportunities for conflicts to occur.

2. 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 packageA and packageB have been pinned to use specific versions (packageA==0.44.1b0 packageB==4.3.0).

To find a version of both packageA and packageB that depend on the same version of packageC, you might consider:

• Loosening the range of packages that you are prepared to install (e.g. pip install "packageA>0.44.*" "packageB>4.0.0")
• Asking pip to install any version of packageA and packageB by removing the version specifiers altogether (e.g. pip install packageA packageB)

In the second case, pip will automatically find a version of both packageA and packageB that depend on the same version of packageC, installing:

• packageA 0.46.0b0, which depends on packageC 2.6.1
• packageB 4.3.0 which also depends on packageC 2.6.1

If you want to prioritise one package over another, you can add version specifiers to only the more important package:

pip install packageA==0.44.1b0 packageB

This will result in:

• packageA 0.44.1b0, which depends on packageC 2.6.1
• packageB 4.1.3 which also depends on packageC 2.6.1

Now that you have resolved the issue, you can repin the compatible package versions as required.

3. Loosen the requirements of your downstream 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 downstream 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!

4. All requirements are loose, 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 discuss
• Python user forums
• Python developers Slack channel
• Python IRC
• Stack Overflow

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

[nlhkabu]

@pfmoore @uranusjr @pradyunsg @ei8fdb
https://hackmd.io/bJ69K88tSa-6wRy4mo6D9A is ready for review - please read and add comments.