For years, pip would silently install the wrong version of a package. If you specified foo == 1.2 and also bar > 2.0, and bar happened to depend on foo > 1.2, pip would happily install foo 1.3. Your explicit pin was quietly discarded. No error. No warning. It just did the wrong thing.

This is the worst kind of failure a package manager can have. An error would be fine. A warning would be tolerable. Silently violating the one constraint the user bothered to write down is inexcusable.

The old “legacy resolver” processed dependencies in order and allowed transitive dependencies to override direct pins. When you told people about this behavior, many of them insisted it could not happen. It could, and it did, routinely.

Pip 20.3, released in November 2020, introduced a new backtracking resolver that properly detects these conflicts. With the new resolver, the foo == 1.2 and foo > 1.2 conflict raises a ResolutionImpossible error instead of silently installing the wrong thing. The legacy resolver lingered behind an opt-in flag until pip 23.1 (April 2023), when it was finally removed.

So the resolver is fixed. But the overall workflow still leaves gaps. You can still end up with an incoherent environment if you use --no-deps, which bypasses the resolver entirely. You can also get there by running multiple pip install invocations with different requirements files, since each invocation only validates the constraints it can see. And pre-existing packages in the environment are not re-validated against new constraints unless they are part of the current resolution.

If you want to verify that your environment is actually consistent after installation, run pip check. It will report any installed packages whose dependencies are unsatisfied or conflicting. That this is not the default behavior is itself a design choice worth questioning.

The resolver took from 2020 to 2023 to fully replace the broken one, and the broken one had been shipping for years before that. Silent failure in dependency resolution is a serious defect, and it persisted for far too long.