FAQ

FAQ #

Why is the dependency resolution process slow? #

While the dependency resolver at the heart of Poetry is highly optimized and should be fast enough for most cases, with certain sets of dependencies it can take time to find a valid solution.

This is due to the fact that not all libraries on PyPI have properly declared their metadata and, as such, they are not available via the PyPI JSON API. At this point, Poetry has no choice but to download the packages and inspect them to get the necessary information. This is an expensive operation, both in bandwidth and time, which is why it seems this is a long process.

At the moment there is no way around it. However, if you notice that Poetry is downloading many versions of a single package, you can lessen the workload by constraining that one package in your pyproject.toml more narrowly. That way Poetry does not have to sift through so many versions of it, which may speed up the locking process considerably in some cases.

What kind of versioning scheme does Poetry use for itself? #

Poetry uses “major.minor.micro” version identifiers as mentioned in PEP 440.

Version bumps are done similar to Python’s versioning:

• A major version bump (incrementing the first number) is only done for breaking changes if a deprecation cycle is not possible and many users have to perform some manual steps to migrate from one version to the next one.
• A minor version bump (incrementing the second number) may include new features as well as new deprecations and drop features deprecated in an earlier minor release.
• A micro version bump (incrementing the third number) usually only includes bug fixes. Deprecated features will not be dropped in a micro release.

Why does Poetry not adhere to semantic versioning? #

Because of its large user base, even small changes not considered relevant by most users can turn out to be a breaking change for some users in hindsight. Sticking to strict semantic versioning and (almost) always bumping the major version instead of the minor version does not seem desirable since the minor version will not carry any meaning anymore.

Are unbound version constraints a bad idea? #

A version constraint without an upper bound such as * or >=3.4 will allow updates to any future version of the dependency. This includes major versions breaking backward compatibility.

Once a release of your package is published, you cannot tweak its dependencies anymore in case a dependency breaks BC – you have to do a new release but the previous one stays broken. (Users can still work around the broken dependency by restricting it by themselves.)

To avoid such issues you can define an upper bound on your constraints, which you can increase in a new release after testing that your package is compatible with the new major version of your dependency.

For example instead of using >=3.4 you can use ^3.4 which allows all versions <4.0. The ^ operator works very well with libraries following semantic versioning.

However, when defining an upper bound, users of your package are not able to update a dependency beyond the upper bound even if it does not break anything and is fully compatible with your package. You have to release a new version of your package with an increased upper bound first.

If your package will be used as a library in other packages, it might be better to avoid upper bounds and thus unnecessary dependency conflicts (unless you already know for sure that the next release of the dependency will break your package). If your package will be used as an application, it might be worth to define an upper bound.

Is tox supported? #

Yes. By using the isolated builds tox provides, you can use it in combination with the PEP 517 compliant build system provided by Poetry.

So, in your pyproject.toml file, add this section if it does not already exist:

[build-system]
requires = ["poetry-core>=1.0.0"]
build-backend = "poetry.core.masonry.api"

tox can be configured in multiple ways. It depends on what should be the code under test and which dependencies should be installed.

Usecase #1 #

[tox]
isolated_build = true

[testenv]
deps =
    pytest
commands =
    pytest tests/ --import-mode importlib

tox will create an sdist package of the project and uses pip to install it in a fresh environment. Thus, dependencies are resolved by pip.

Usecase #2 #

[tox]
isolated_build = true

[testenv]
allowlist_externals = poetry
commands_pre =
    poetry install --no-root --sync
commands =
    poetry run pytest tests/ --import-mode importlib

tox will create an sdist package of the project and uses pip to install it in a fresh environment. Thus, dependencies are resolved by pip in the first place. But afterwards we run Poetry, which will install the locked dependencies into the environment.

Usecase #3 #

[tox]
isolated_build = true

[testenv]
skip_install = true
allowlist_externals = poetry
commands_pre =
    poetry install
commands =
    poetry run pytest tests/ --import-mode importlib

tox will not do any install. Poetry installs all the dependencies and the current package an editable mode. Thus, tests are running against the local files and not the built and installed package.

I don’t want Poetry to manage my virtual environments. Can I disable it? #

While Poetry automatically creates virtual environments to always work isolated from the global Python installation, there are valid reasons why it’s not necessary and is an overhead, like when working with containers.

In this case, you can disable this feature by setting the virtualenvs.create setting to false:

poetry config virtualenvs.create false

Why is Poetry telling me that the current project’s Python requirement is not compatible with one or more packages' Python requirements? #

Unlike pip, Poetry doesn’t resolve for just the Python in the current environment. Instead it makes sure that a dependency is resolvable within the given Python version range in pyproject.toml.

Assume you have the following pyproject.toml:

[tool.poetry.dependencies]
python = "^3.7"

This means your project aims to be compatible with any Python version >=3.7,<4.0. Whenever you try to add a dependency whose Python requirement doesn’t match the whole range Poetry will tell you this, e.g.:

The current project's Python requirement (>=3.7.0,<4.0.0) is not compatible with some of the required packages Python requirement:
    - scipy requires Python >=3.7,<3.11, so it will not be satisfied for Python >=3.11,<4.0.0

Usually you will want to match the Python requirement of your project with the upper bound of the failing dependency. Alternative you can tell Poetry to install this dependency only for a specific range of Python versions, if you know that it’s not needed in all versions.

Why does Poetry enforce PEP 440 versions? #

This is done so to be compliant with the broader Python ecosystem.

For example, if Poetry builds a distribution for a project that uses a version that is not valid according to PEP 440, third party tools will be unable to parse the version correctly.

Poetry busts my Docker cache because it requires me to COPY my source files in before installing 3rd party dependencies #

By default running poetry install ... requires you to have your source files present (both the “root” package and any directory path dependencies you might have). This interacts poorly with Docker’s caching mechanisms because any change to a source file will make any layers (subsequent commands in your Dockerfile) re-run. For example, you might have a Dockerfile that looks something like this:

FROM python
COPY pyproject.toml poetry.lock .
COPY src/ ./src
RUN pip install poetry && poetry install --no-dev

As soon as any source file changes, the cache for the RUN layer will be invalidated, which forces all 3rd party dependencies (likely the slowest step out of these) to be installed again if you changed any files in src/.

To avoid this cache busting you can split this into two steps:

1. Install 3rd party dependencies.
2. Copy over your source code and install just the source code.

This might look something like this:

FROM python
COPY pyproject.toml poetry.lock .
RUN pip install poetry && poetry install --no-root --no-directory
COPY src/ ./src
RUN poetry install --no-dev

The two key options we are using here are --no-root (skips installing the project source) and --no-directory (skips installing any local directory path dependencies, you can omit this if you don’t have any). More information on the options available for poetry install.