# Managing dependencies

## Dependency fields

Dependencies of the project are defined in several fields:

- [`project.dependencies`](#project-dependencies): Published dependencies.
- [`project.optional-dependencies`](#optional-dependencies): Published optional dependencies, or
  "extras".
- [`dependency-groups`](#dependency-groups): Local dependencies for development.
- [`tool.uv.sources`](#dependency-sources): Alternative sources for dependencies during development.

!!! note

    The `project.dependencies` and `project.optional-dependencies` fields can be used even if
    project isn't going to be published. `dependency-groups` are a recently standardized feature
    and may not be supported by all tools yet.

uv supports modifying the project's dependencies with `uv add` and `uv remove`, but dependency
metadata can also be updated by editing the `pyproject.toml` directly.

## Adding dependencies

To add a dependency:

```console
$ uv add httpx
```

An entry will be added in the `project.dependencies` field:

```toml title="pyproject.toml" hl_lines="4"
[project]
name = "example"
version = "0.1.0"
dependencies = ["httpx>=0.27.2"]
```

The [`--dev`](#development-dependencies), [`--group`](#dependency-groups), or
[`--optional`](#optional-dependencies) flags can be used to add dependencies to an alternative
field.

The dependency will include a constraint, e.g., `>=0.27.2`, for the most recent, compatible version
of the package. The kind of bound can be adjusted with
[`--bounds`](../../reference/settings.md#add-bounds), or the constraint can be provided directly:

```console
$ uv add "httpx>=0.20"
```

When adding a dependency from a source other than a package registry, uv will add an entry in the
sources field. For example, when adding `httpx` from GitHub:

```console
$ uv add "httpx @ git+https://github.com/encode/httpx"
```

The `pyproject.toml` will include a [Git source entry](#git):

```toml title="pyproject.toml" hl_lines="8-9"
[project]
name = "example"
version = "0.1.0"
dependencies = [
    "httpx",
]

[tool.uv.sources]
httpx = { git = "https://github.com/encode/httpx" }
```

If a dependency cannot be used, uv will display an error.:

```console
$ uv add "httpx>9999"
  × No solution found when resolving dependencies:
  ╰─▶ Because only httpx<=1.0.0b0 is available and your project depends on httpx>9999,
      we can conclude that your project's requirements are unsatisfiable.
```

### Importing dependencies from requirements files

Dependencies declared in a `requirements.txt` file can be added to the project with the `-r` option:

```
uv add -r requirements.txt
```

See the [pip migration guide](../../guides/migration/pip-to-project.md#importing-requirements-files)
for more details.

## Removing dependencies

To remove a dependency:

```console
$ uv remove httpx
```

The `--dev`, `--group`, or `--optional` flags can be used to remove a dependency from a specific
table.

If a [source](#dependency-sources) is defined for the removed dependency, and there are no other
references to the dependency, it will also be removed.

## Changing dependencies

To change an existing dependency, e.g., to use a different constraint for `httpx`:

```console
$ uv add "httpx>0.1.0"
```

!!! note

    In this example, we are changing the constraints for the dependency in the `pyproject.toml`.
    The locked version of the dependency will only change if necessary to satisfy the new
    constraints. To force the package version to update to the latest within the constraints, use `--upgrade-package <name>`, e.g.:

    ```console
    $ uv add "httpx>0.1.0" --upgrade-package httpx
    ```

    See the [lockfile](./sync.md#upgrading-locked-package-versions) documentation for more details
    on upgrading packages.

Requesting a different dependency source will update the `tool.uv.sources` table, e.g., to use
`httpx` from a local path during development:

```console
$ uv add "httpx @ ../httpx"
```

## Platform-specific dependencies

To ensure that a dependency is only installed on a specific platform or on specific Python versions,
use [environment markers](https://peps.python.org/pep-0508/#environment-markers).

For example, to install `jax` on Linux, but not on Windows or macOS:

```console
$ uv add "jax; sys_platform == 'linux'"
```

The resulting `pyproject.toml` will then include the environment marker in the dependency
definition:

```toml title="pyproject.toml" hl_lines="6"
[project]
name = "project"
version = "0.1.0"
requires-python = ">=3.11"
dependencies = ["jax; sys_platform == 'linux'"]
```

Similarly, to include `numpy` on Python 3.11 and later:

```console
$ uv add "numpy; python_version >= '3.11'"
```

See Python's [environment marker](https://peps.python.org/pep-0508/#environment-markers)
documentation for a complete enumeration of the available markers and operators.

!!! tip

    Dependency sources can also be [changed per-platform](#platform-specific-sources).

## Project dependencies

The `project.dependencies` table represents the dependencies that are used when uploading to PyPI or
building a wheel. Individual dependencies are specified using
[dependency specifiers](https://packaging.python.org/en/latest/specifications/dependency-specifiers/)
syntax, and the table follows the
[PEP 621](https://packaging.python.org/en/latest/specifications/pyproject-toml/) standard.

`project.dependencies` defines the list of packages that are required for the project, along with
the version constraints that should be used when installing them. Each entry includes a dependency
name and version. An entry may include extras or environment markers for platform-specific packages.
For example:

```toml title="pyproject.toml"
[project]
name = "albatross"
version = "0.1.0"
dependencies = [
  # Any version in this range
  "tqdm >=4.66.2,<5",
  # Exactly this version of torch
  "torch ==2.2.2",
  # Install transformers with the torch extra
  "transformers[torch] >=4.39.3,<5",
  # Only install this package on older python versions
  # See "Environment Markers" for more information
  "importlib_metadata >=7.1.0,<8; python_version < '3.10'",
  "mollymawk ==0.1.0"
]
```

## Dependency sources

The `tool.uv.sources` table extends the standard dependency tables with alternative dependency
sources, which are used during development.

Dependency sources add support for common patterns that are not supported by the
`project.dependencies` standard, like editable installations and relative paths. For example, to
install `foo` from a directory relative to the project root:

```toml title="pyproject.toml" hl_lines="7"
[project]
name = "example"
version = "0.1.0"
dependencies = ["foo"]

[tool.uv.sources]
foo = { path = "./packages/foo" }
```

The following dependency sources are supported by uv:

- [Index](#index): A package resolved from a specific package index.
- [Git](#git): A Git repository.
- [URL](#url): A remote wheel or source distribution.
- [Path](#path): A local wheel, source distribution, or project directory.
- [Workspace](#workspace-member): A member of the current workspace.

!!! important

    Sources are only respected by uv. If another tool is used, only the definitions in the standard
    project tables will be used. If another tool is being used for development, any metadata
    provided in the source table will need to be re-specified in the other tool's format.

### Index

To add Python package from a specific index, use the `--index` option:

```console
$ uv add torch --index pytorch=https://download.pytorch.org/whl/cpu
```

uv will store the index in `[[tool.uv.index]]` and add a `[tool.uv.sources]` entry:

```toml title="pyproject.toml"
[project]
dependencies = ["torch"]

[tool.uv.sources]
torch = { index = "pytorch" }

[[tool.uv.index]]
name = "pytorch"
url = "https://download.pytorch.org/whl/cpu"
```

!!! tip

    The above example will only work on x86-64 Linux, due to the specifics of the PyTorch index.
    See the [PyTorch guide](../../guides/integration/pytorch.md) for more information about setting
    up PyTorch.

Using an `index` source _pins_ a package to the given index — it will not be downloaded from other
indexes.

When defining an index, an `explicit` flag can be included to indicate that the index should _only_
be used for packages that explicitly specify it in `tool.uv.sources`. If `explicit` is not set,
other packages may be resolved from the index, if not found elsewhere.

```toml title="pyproject.toml" hl_lines="4"
[[tool.uv.index]]
name = "pytorch"
url = "https://download.pytorch.org/whl/cpu"
explicit = true
```

### Git

To add a Git dependency source, prefix a Git-compatible URL with `git+`.

For example:

```console
$ # Install over HTTP(S).
$ uv add git+https://github.com/encode/httpx

$ # Install over SSH.
$ uv add git+ssh://git@github.com/encode/httpx
```

```toml title="pyproject.toml" hl_lines="5"
[project]
dependencies = ["httpx"]

[tool.uv.sources]
httpx = { git = "https://github.com/encode/httpx" }
```

Specific Git references can be requested, e.g., a tag:

```console
$ uv add git+https://github.com/encode/httpx --tag 0.27.0
```

```toml title="pyproject.toml" hl_lines="7"
[project]
dependencies = ["httpx"]

[tool.uv.sources]
httpx = { git = "https://github.com/encode/httpx", tag = "0.27.0" }
```

Or, a branch:

```console
$ uv add git+https://github.com/encode/httpx --branch main
```

```toml title="pyproject.toml" hl_lines="7"
[project]
dependencies = ["httpx"]

[tool.uv.sources]
httpx = { git = "https://github.com/encode/httpx", branch = "main" }
```

Or, a revision (commit):

```console
$ uv add git+https://github.com/encode/httpx --rev 326b9431c761e1ef1e00b9f760d1f654c8db48c6
```

```toml title="pyproject.toml" hl_lines="7"
[project]
dependencies = ["httpx"]

[tool.uv.sources]
httpx = { git = "https://github.com/encode/httpx", rev = "326b9431c761e1ef1e00b9f760d1f654c8db48c6" }
```

A `subdirectory` may be specified if the package isn't in the repository root:

```console
$ uv add git+https://github.com/langchain-ai/langchain#subdirectory=libs/langchain
```

```toml title="pyproject.toml"
[project]
dependencies = ["langchain"]

[tool.uv.sources]
langchain = { git = "https://github.com/langchain-ai/langchain", subdirectory = "libs/langchain" }
```

### URL

To add a URL source, provide a `https://` URL to either a wheel (ending in `.whl`) or a source
distribution (typically ending in `.tar.gz` or `.zip`; see
[here](../../concepts/resolution.md#source-distribution) for all supported formats).

For example:

```console
$ uv add "https://files.pythonhosted.org/packages/5c/2d/3da5bdf4408b8b2800061c339f240c1802f2e82d55e50bd39c5a881f47f0/httpx-0.27.0.tar.gz"
```

Will result in a `pyproject.toml` with:

```toml title="pyproject.toml" hl_lines="5"
[project]
dependencies = ["httpx"]

[tool.uv.sources]
httpx = { url = "https://files.pythonhosted.org/packages/5c/2d/3da5bdf4408b8b2800061c339f240c1802f2e82d55e50bd39c5a881f47f0/httpx-0.27.0.tar.gz" }
```

URL dependencies can also be manually added or edited in the `pyproject.toml` with the
`{ url = <url> }` syntax. A `subdirectory` may be specified if the source distribution isn't in the
archive root.

### Path

To add a path source, provide the path of a wheel (ending in `.whl`), a source distribution
(typically ending in `.tar.gz` or `.zip`; see
[here](../../concepts/resolution.md#source-distribution) for all supported formats), or a directory
containing a `pyproject.toml`.

For example:

```console
$ uv add /example/foo-0.1.0-py3-none-any.whl
```

Will result in a `pyproject.toml` with:

```toml title="pyproject.toml"
[project]
dependencies = ["foo"]

[tool.uv.sources]
foo = { path = "/example/foo-0.1.0-py3-none-any.whl" }
```

The path may also be a relative path:

```console
$ uv add ./foo-0.1.0-py3-none-any.whl
```

Or, a path to a project directory:

```console
$ uv add ~/projects/bar/
```

!!! important

    When using a directory as a path dependency, uv will attempt to build and install the target as
    a package by default. See the [virtual dependency](#virtual-dependencies) documentation for
    details.

An [editable installation](#editable-dependencies) is not used for path dependencies by default. An
editable installation may be requested for project directories:

```console
$ uv add --editable ../projects/bar/
```

Which will result in a `pyproject.toml` with:

```toml title="pyproject.toml"
[project]
dependencies = ["bar"]

[tool.uv.sources]
bar = { path = "../projects/bar", editable = true }
```

!!! tip

    For multiple packages in the same repository, [_workspaces_](./workspaces.md) may be a better
    fit.

### Workspace member

To declare a dependency on a workspace member, add the member name with `{ workspace = true }`. All
workspace members must be explicitly stated. Workspace members are always
[editable](#editable-dependencies) . See the [workspace](./workspaces.md) documentation for more
details on workspaces.

```toml title="pyproject.toml"
[project]
dependencies = ["foo==0.1.0"]

[tool.uv.sources]
foo = { workspace = true }

[tool.uv.workspace]
members = [
  "packages/foo"
]
```

### Platform-specific sources

You can limit a source to a given platform or Python version by providing
[dependency specifiers](https://packaging.python.org/en/latest/specifications/dependency-specifiers/)-compatible
environment markers for the source.

For example, to pull `httpx` from GitHub, but only on macOS, use the following:

```toml title="pyproject.toml" hl_lines="8"
[project]
dependencies = ["httpx"]

[tool.uv.sources]
httpx = { git = "https://github.com/encode/httpx", tag = "0.27.2", marker = "sys_platform == 'darwin'" }
```

By specifying the marker on the source, uv will still include `httpx` on all platforms, but will
download the source from GitHub on macOS, and fall back to PyPI on all other platforms.

### Multiple sources

You can specify multiple sources for a single dependency by providing a list of sources,
disambiguated by [PEP 508](https://peps.python.org/pep-0508/#environment-markers)-compatible
environment markers.

For example, to pull in different `httpx` tags on macOS vs. Linux:

```toml title="pyproject.toml" hl_lines="6-7"
[project]
dependencies = ["httpx"]

[tool.uv.sources]
httpx = [
  { git = "https://github.com/encode/httpx", tag = "0.27.2", marker = "sys_platform == 'darwin'" },
  { git = "https://github.com/encode/httpx", tag = "0.24.1", marker = "sys_platform == 'linux'" },
]
```

This strategy extends to using different indexes based on environment markers. For example, to
install `torch` from different PyTorch indexes based on the platform:

```toml title="pyproject.toml" hl_lines="6-7"
[project]
dependencies = ["torch"]

[tool.uv.sources]
torch = [
  { index = "torch-cpu", marker = "platform_system == 'Darwin'"},
  { index = "torch-gpu", marker = "platform_system == 'Linux'"},
]

[[tool.uv.index]]
name = "torch-cpu"
url = "https://download.pytorch.org/whl/cpu"
explicit = true

[[tool.uv.index]]
name = "torch-gpu"
url = "https://download.pytorch.org/whl/cu124"
explicit = true
```

### Disabling sources

To instruct uv to ignore the `tool.uv.sources` table (e.g., to simulate resolving with the package's
published metadata), use the `--no-sources` flag:

```console
$ uv lock --no-sources
```

The use of `--no-sources` will also prevent uv from discovering any
[workspace members](#workspace-member) that could satisfy a given dependency.

## Optional dependencies

It is common for projects that are published as libraries to make some features optional to reduce
the default dependency tree. For example, Pandas has an
[`excel` extra](https://pandas.pydata.org/docs/getting_started/install.html#excel-files) and a
[`plot` extra](https://pandas.pydata.org/docs/getting_started/install.html#visualization) to avoid
installation of Excel parsers and `matplotlib` unless someone explicitly requires them. Extras are
requested with the `package[<extra>]` syntax, e.g., `pandas[plot, excel]`.

Optional dependencies are specified in `[project.optional-dependencies]`, a TOML table that maps
from extra name to its dependencies, following [dependency specifiers](#dependency-specifiers)
syntax.

Optional dependencies can have entries in `tool.uv.sources` the same as normal dependencies.

```toml title="pyproject.toml"
[project]
name = "pandas"
version = "1.0.0"

[project.optional-dependencies]
plot = [
  "matplotlib>=3.6.3"
]
excel = [
  "odfpy>=1.4.1",
  "openpyxl>=3.1.0",
  "python-calamine>=0.1.7",
  "pyxlsb>=1.0.10",
  "xlrd>=2.0.1",
  "xlsxwriter>=3.0.5"
]
```

To add an optional dependency, use the `--optional <extra>` option:

```console
$ uv add httpx --optional network
```

!!! note

    If you have optional dependencies that conflict with one another, resolution will fail
    unless you explicitly [declare them as conflicting](./config.md#conflicting-dependencies).

Sources can also be declared as applying only to a specific optional dependency. For example, to
pull `torch` from different PyTorch indexes based on an optional `cpu` or `gpu` extra:

```toml title="pyproject.toml"
[project]
dependencies = []

[project.optional-dependencies]
cpu = [
  "torch",
]
gpu = [
  "torch",
]

[tool.uv.sources]
torch = [
  { index = "torch-cpu", extra = "cpu" },
  { index = "torch-gpu", extra = "gpu" },
]

[[tool.uv.index]]
name = "torch-cpu"
url = "https://download.pytorch.org/whl/cpu"

[[tool.uv.index]]
name = "torch-gpu"
url = "https://download.pytorch.org/whl/cu124"
```

## Development dependencies

Unlike optional dependencies, development dependencies are local-only and will _not_ be included in
the project requirements when published to PyPI or other indexes. As such, development dependencies
are not included in the `[project]` table.

Development dependencies can have entries in `tool.uv.sources` the same as normal dependencies.

To add a development dependency, use the `--dev` flag:

```console
$ uv add --dev pytest
```

uv uses the `[dependency-groups]` table (as defined in [PEP 735](https://peps.python.org/pep-0735/))
for declaration of development dependencies. The above command will create a `dev` group:

```toml title="pyproject.toml"
[dependency-groups]
dev = [
  "pytest >=8.1.1,<9"
]
```

The `dev` group is special-cased; there are `--dev`, `--only-dev`, and `--no-dev` flags to toggle
inclusion or exclusion of its dependencies. See `--no-default-groups` to disable all default groups
instead. Additionally, the `dev` group is [synced by default](#default-groups).

### Dependency groups

Development dependencies can be divided into multiple groups, using the `--group` flag.

For example, to add a development dependency in the `lint` group:

```console
$ uv add --group lint ruff
```

Which results in the following `[dependency-groups]` definition:

```toml title="pyproject.toml"
[dependency-groups]
dev = [
  "pytest"
]
lint = [
  "ruff"
]
```

Once groups are defined, the `--all-groups`, `--no-default-groups`, `--group`, `--only-group`, and
`--no-group` options can be used to include or exclude their dependencies.

!!! tip

    The `--dev`, `--only-dev`, and `--no-dev` flags are equivalent to `--group dev`,
    `--only-group dev`, and `--no-group dev` respectively.

uv requires that all dependency groups are compatible with each other and resolves all groups
together when creating the lockfile.

If dependencies declared in one group are not compatible with those in another group, uv will fail
to resolve the requirements of the project with an error.

!!! note

    If you have dependency groups that conflict with one another, resolution will fail
    unless you explicitly [declare them as conflicting](./config.md#conflicting-dependencies).

### Nesting groups

A dependency group can include other dependency groups, e.g.:

```toml title="pyproject.toml"
[dependency-groups]
dev = [
  {include-group = "lint"},
  {include-group = "test"}
]
lint = [
  "ruff"
]
test = [
  "pytest"
]
```

An included group's dependencies cannot conflict with the other dependencies declared in a group.

### Default groups

By default, uv includes the `dev` dependency group in the environment (e.g., during `uv run` or
`uv sync`). The default groups to include can be changed using the `tool.uv.default-groups` setting.

```toml title="pyproject.toml"
[tool.uv]
default-groups = ["dev", "foo"]
```

To enable all dependencies groups by default, use `"all"` instead of listing group names:

```toml title="pyproject.toml"
[tool.uv]
default-groups = "all"
```

!!! tip

    To disable this behaviour during `uv run` or `uv sync`, use `--no-default-groups`.
    To exclude a specific default group, use `--no-group <name>`.

### Group `requires-python`

By default, dependency groups must be compatible with your project's `requires-python` range.

If a dependency group requires a different range of Python versions than your project, you can
specify a `requires-python` for the group in `[tool.uv.dependency-groups]`, e.g.:

```toml title="pyproject.toml" hl_lines="9-10"
[project]
name = "example"
version = "0.0.0"
requires-python = ">=3.10"

[dependency-groups]
dev = ["pytest"]

[tool.uv.dependency-groups]
dev = {requires-python = ">=3.12"}
```

### Legacy `dev-dependencies`

Before `[dependency-groups]` was standardized, uv used the `tool.uv.dev-dependencies` field to
specify development dependencies, e.g.:

```toml title="pyproject.toml"
[tool.uv]
dev-dependencies = [
  "pytest"
]
```

Dependencies declared in this section will be combined with the contents in the
`dependency-groups.dev`. Eventually, the `dev-dependencies` field will be deprecated and removed.

!!! note

    If a `tool.uv.dev-dependencies` field exists, `uv add --dev` will use the existing section
    instead of adding a new `dependency-groups.dev` section.

## Build dependencies

If a project is structured as [Python package](./config.md#build-systems), it may declare
dependencies that are required to build the project, but not required to run it. These dependencies
are specified in the `[build-system]` table under `build-system.requires`, following
[PEP 518](https://peps.python.org/pep-0518/).

For example, if a project uses `setuptools` as its build backend, it should declare `setuptools` as
a build dependency:

```toml title="pyproject.toml"
[project]
name = "pandas"
version = "0.1.0"

[build-system]
requires = ["setuptools>=42"]
build-backend = "setuptools.build_meta"
```

By default, uv will respect `tool.uv.sources` when resolving build dependencies. For example, to use
a local version of `setuptools` for building, add the source to `tool.uv.sources`:

```toml title="pyproject.toml"
[project]
name = "pandas"
version = "0.1.0"

[build-system]
requires = ["setuptools>=42"]
build-backend = "setuptools.build_meta"

[tool.uv.sources]
setuptools = { path = "./packages/setuptools" }
```

When publishing a package, we recommend running `uv build --no-sources` to ensure that the package
builds correctly when `tool.uv.sources` is disabled, as is the case when using other build tools,
like [`pypa/build`](https://github.com/pypa/build).

## Editable dependencies

A regular installation of a directory with a Python package first builds a wheel and then installs
that wheel into your virtual environment, copying all source files. When the package source files
are edited, the virtual environment will contain outdated versions.

Editable installations solve this problem by adding a link to the project within the virtual
environment (a `.pth` file), which instructs the interpreter to include the source files directly.

There are some limitations to editables (mainly: the build backend needs to support them, and native
modules aren't recompiled before import), but they are useful for development, as the virtual
environment will always use the latest changes to the package.

uv uses editable installation for workspace packages by default.

To add an editable dependency, use the `--editable` flag:

```console
$ uv add --editable ./path/foo
```

Or, to opt-out of using an editable dependency in a workspace:

```console
$ uv add --no-editable ./path/foo
```

## Virtual dependencies

uv allows dependencies to be "virtual", in which the dependency itself is not installed as a
[package](./config.md#project-packaging), but its dependencies are.

By default, dependencies are never virtual.

A dependency with a [`path` source](#path) can be virtual if it explicitly sets
[`tool.uv.package = false`](../../reference/settings.md#package). Unlike working _in_ the dependent
project with uv, the package will be built even if a [build system](./config.md#build-systems) is
not declared.

To treat a dependency as virtual, set `package = false` on the source:

```toml title="pyproject.toml"
[project]
dependencies = ["bar"]

[tool.uv.sources]
bar = { path = "../projects/bar", package = false }
```

If a dependency sets `tool.uv.package = false`, it can be overridden by declaring `package = true`
on the source:

```toml title="pyproject.toml"
[project]
dependencies = ["bar"]

[tool.uv.sources]
bar = { path = "../projects/bar", package = true }
```

Similarly, a dependency with a [`workspace` source](#workspace-member) can be virtual if it
explicitly sets [`tool.uv.package = false`](../../reference/settings.md#package). The workspace
member will be built even if a [build system](./config.md#build-systems) is not declared.

Workspace members that are _not_ dependencies can be virtual by default, e.g., if the parent
`pyproject.toml` is:

```toml title="pyproject.toml"
[project]
name = "parent"
version = "1.0.0"
dependencies = []

[tool.uv.workspace]
members = ["child"]
```

And the child `pyproject.toml` excluded a build system:

```toml title="pyproject.toml"
[project]
name = "child"
version = "1.0.0"
dependencies = ["anyio"]
```

Then the `child` workspace member would not be installed, but the transitive dependency `anyio`
would be.

In contrast, if the parent declared a dependency on `child`:

```toml title="pyproject.toml"
[project]
name = "parent"
version = "1.0.0"
dependencies = ["child"]

[tool.uv.sources]
child = { workspace = true }

[tool.uv.workspace]
members = ["child"]
```

Then `child` would be built and installed.

## Dependency specifiers

uv uses standard
[dependency specifiers](https://packaging.python.org/en/latest/specifications/dependency-specifiers/),
originally defined in [PEP 508](https://peps.python.org/pep-0508/). A dependency specifier is
composed of, in order:

- The dependency name
- The extras you want (optional)
- The version specifier
- An environment marker (optional)

The version specifiers are comma separated and added together, e.g., `foo >=1.2.3,<2,!=1.4.0` is
interpreted as "a version of `foo` that's at least 1.2.3, but less than 2, and not 1.4.0".

Specifiers are padded with trailing zeros if required, so `foo ==2` matches foo 2.0.0, too.

A star can be used for the last digit with equals, e.g., `foo ==2.1.*` will accept any release from
the 2.1 series. Similarly, `~=` matches where the last digit is equal or higher, e.g., `foo ~=1.2`
is equal to `foo >=1.2,<2`, and `foo ~=1.2.3` is equal to `foo >=1.2.3,<1.3`.

Extras are comma-separated in square bracket between name and version, e.g.,
`pandas[excel,plot] ==2.2`. Whitespace between extra names is ignored.

Some dependencies are only required in specific environments, e.g., a specific Python version or
operating system. For example to install the `importlib-metadata` backport for the
`importlib.metadata` module, use `importlib-metadata >=7.1.0,<8; python_version < '3.10'`. To
install `colorama` on Windows (but omit it on other platforms), use
`colorama >=0.4.6,<5; platform_system == "Windows"`.

Markers are combined with `and`, `or`, and parentheses, e.g.,
`aiohttp >=3.7.4,<4; (sys_platform != 'win32' or implementation_name != 'pypy') and python_version >= '3.10'`.
Note that versions within markers must be quoted, while versions _outside_ of markers must _not_ be
quoted.
