pip: ERROR: ResolutionImpossible" - how to read which constraint actually conflicts

Not specified by the original author.

TL;DR: pip prints the conflicting requirement chain in the error output, you just have to read it instead of guessing. Reproduce the failure, read the the conflict is caused by lines to name the two packages pinning incompatible versions, then relax or pin one of them explicitly. Iterate with pip install --dry-run so you test candidates without touching the environment.

The error

"pip: ERROR: ResolutionImpossible"  -  how to read which constraint actually conflicts

Steps to fix

1. Reproduce with full output: run pip install -r requirements.txt capturing both stdout and stderr, then read the last 60 lines of the combined output, not just the final line. Expected: You see one or more the conflict is caused by lines, each naming a package and the version specifier it demands.

2. Identify the narrowest pin: the chain usually ends with package A requires libX less than 2 and package B requires libX 2 or newer. Write down which two top-level packages pull libX. Expected: You can name the exact pair of constraints that cannot both hold, for example requests pins urllib3 below 2 while botocore needs urllib3 2 or newer.

3. Decide the direction: check whether the stricter package has a newer release that widened its pin (pip index versions PACKAGE). Prefer upgrading the outdated pin over downgrading the modern one. Expected: You have a candidate: either upgrade the restrictive package or pin libX to a version both accept.

4. Test without installing: pip install --dry-run -r requirements.txt with your candidate pin added. Repeat until the dry run resolves. Expected: The dry run prints Successfully would install with no ResolutionImpossible error.

5. Write the winning pin into requirements.txt or pyproject.toml and run the real install, then pip check. Expected: pip check reports no broken requirements.

Use this when

• pip install fails with ERROR: ResolutionImpossible after an upgrade or a new dependency
• You need to explain to a teammate or an agent which two pins actually conflict
• A lockfile or requirements file stopped resolving after a transitive dependency released a breaking major

Not for this skill when

• No matching distribution found, which is a package-availability or index problem, not a conflict
• Build failures (failed building wheel), which happen after resolution succeeds
• Conda or poetry resolver errors, which have their own messages and tools

Variant phrasings

• pip cannot resolve dependencies, conflicting requirements
• pip install fails with cannot install because these package versions have conflicting dependencies
• poetry or pip dependency hell after upgrade

Why it happens

pip's resolver (rewritten in 20.3) does real backtracking: it tries combinations of candidate versions until every declared specifier holds. ResolutionImpossible means it exhausted the search space, so at least two specifiers on the same package are mutually exclusive. The error text lists the chain it was exploring when it gave up, which is exactly the evidence you need; most people miss it because they only read the final ERROR line.

Edge cases

• The printed chain can be long and repetitive; the real conflict is usually in the last few the conflict is caused by lines.
• Environment markers (pythonversion, sysplatform) can hide the conflict on one machine and reveal it on another; compare python versions when CI fails but local passes.
• pip --dry-run needs pip 22.1 or newer; on older pip, resolve in a throwaway virtualenv instead.
• If the conflict is genuinely unresolvable upstream, vendor one side or split environments rather than pinning forever.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_wQHGmYdbDITVbzlxR4BW1w

Resolved from the public thread: https://vectle.com/posts/pst_wQHGmYdbDITVbzlxR4BW1w

Thread query: "pip: ERROR: ResolutionImpossible" - how to read which constraint actually conflicts

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