`pip install --dry-run {package}`

[s-m-e]

What's the problem this feature will solve?

In certain scenarios I want to know whether pip could actually install a package and its dependencies - without modifying my Python installation at all.

Some of the related questions:

• Can the dependency tree be fully resolved?
• Are there conflicts?
• Are all required packages available (in currently accessible online sources or local cache)?
• Are packages involved which are not available as wheels and could therefore require a compiler etc. to work (i.e. is there a risk that the installation can fail because of missing/broken non-python tooling)?
• Is the installation folder writable by the respective user, i.e. can the Python environment be modified?

Describe the solution you'd like

I want to be able to run pip install --dry-run {package}. The exit code of the process should indicate "should work" or "probably does not work". The output should provide detailed information.

Ideally, pip performs every operation necessary to install the package - except ultimately changing the current environment. It may (and should) therefore download dependencies, build wheels where applicable, etc.

Real world uses: Similar to plenty of other package management tools which do implement this feature, I see the primary use in head-less, automated installs - i.e. where pip is not triggered by a user but by other tools and software packages. This feature would allow a much more graceful (in advance) handling of failures.

Alternative Solutions

There are plenty of bad and/or risky and/or notoriously unreliable and/or resource-intensive workarounds. Examples:

• Using pip-sync -n (i.e. a third party tool) which has a few massive limitations here
• Using hacks like pip freeze | grep -f requirements.txt | sort | diff -y --suppress-common-lines - <(sort requirements.txt) which assume that you have a requirements.txt file available - again plenty of problems and unsolvable edge cases
• Temporarily creating a new Python (virtual) environment (ideally cloning the target environment somehow), then installing into the temporary environment, then analyzing the outcome (and the difference between the temporary and the target environment).

Additional context

This topic is not new. For reference:

• StackOverflow: How to make pip “dry-run”?
• An old pip issue: deprecate pip install --no-install / pip install --no-download
• (One of a few) attempt(s) to try and implement this feature

[sbidoul]

This sounds like a good feature request and one I have been pondering about too.

A slightly different way to frame it is to say "--dry-run" should output the list of transactions that pip install would apply to the database of installed distributions (PEP 376). I.e. which packages would be added, upgraded, and possibly left unchanged.

A machine readable output would enable higher level use cases such as the creation of a lockfile from a list of top level requirements (à la pip-compile), by running it in an empty environment.

Such a use case does not necessarily require building of all dependencies (only obtaining their metadata). So full build of dependencies could be an option.

[uranusjr]

A problem with --dry-run is that pip can’t really run an installation truly “dry”, since it always needs to build source and sdist. Something like pip install --no-install might make more sense technically, but can be nonsensical to read.

[s-m-e]

@uranusjr Building source and sdist should definitely be part of a proper "dry run". If it fails, it means that the dry run fails meaning that an actual "wet" installation would also fail. This is highly valuable information though I'd also agree with @sbidoul that it could be made optional (e.g. --dry-run --no-dry-builds).

[pradyunsg]

I'm pretty sure there's an existing issue for this feature request.

[pradyunsg]

Closing as a duplicate of #53. That issue can use a cleanup; so I'll do some "housekeeping" on that issue. FWIW, there's also #7819, which is actually an implemented proposal for adding a new pip resolve command.

[pradyunsg]

Okay, housekeeping done. If someone thinks what I did there was inappropriate, please holler here. :P