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FLIT(1) Flit FLIT(1)


flit - Flit Documentation

Flit is a simple way to put Python packages and modules on PyPI. It tries to require less thought about packaging and help you avoid common mistakes. See Why use Flit? <> for more about how it compares to other Python packaging tools.


$ python3 -m pip install flit

Flit requires Python 3 and therefore needs to be installed using the Python 3 version of pip.

Python 2 modules can be distributed using Flit, but need to be importable on Python 3 without errors.


Say you're writing a module foobar — either as a single file, or as a directory — and you want to distribute it.

Make sure that foobar's docstring starts with a one-line summary of what the module is, and that it has a __version__:

"""An amazing sample package!"""
__version__ = "0.1"

Install flit if you don't already have it:

python3 -m pip install flit

Run flit init in the directory containing the module to create a pyproject.toml file. It will look something like this:

requires = ["flit_core >=3.2,<4"]
build-backend = "flit_core.buildapi"
name = "foobar"
authors = [{name = "Sir Robin", email = ""}]
dynamic = ["version", "description"]
Home = ""

You can edit this file to add other metadata, for example to set up command line scripts. See the pyproject.toml page <> of the documentation.

If you have already got a flit.ini file to use with older versions of Flit, convert it to pyproject.toml by running python3 -m flit.tomlify.

Run this command to upload your code to PyPI:

flit publish

Once your package is published, people can install it using pip just like any other package. In most cases, pip will download a 'wheel' package, a standard format it knows how to install. If you specifically ask pip to install an 'sdist' package, it will install and use Flit in a temporary environment.

To install a package locally for development, run:

flit install [--symlink] [--python path/to/python]

Flit packages a single importable module or package at a time, using the import name as the name on PyPI. All subpackages and data files within a package are included automatically.


The pyproject.toml config file

This file lives next to the module or package.


Older version of Flit (up to 0.11) used a flit.ini file for similar information. These files no longer work with Flit 3 and above.

Run python3 -m flit.tomlify to convert a flit.ini file to pyproject.toml.

Build system section

This tells tools like pip to build your project with flit. It's a standard defined by PEP 517. For any new project using Flit, it will look like this:

requires = ["flit_core >=3.2,<4"]
build-backend = "flit_core.buildapi"

Version constraints:

  • For now, all packages should specify <4, so they won't be impacted by changes in the next major version.
  • New style metadata requires flit_core >=3.2
  • Old style metadata requires flit_core >=2,<4
  • The older flit.ini file requires flit_core <3.
  • TOML features new in version 1.0 require flit_core >=3.4.
  • flit_core 3.3 is the last version supporting Python 3.4 & 3.5. Packages supporting these Python versions can only use TOML v0.5 <>.
  • Only flit_core 2.x can build packages on Python 2, so packages still supporting Python 2 cannot use new-style metadata (the [project] table).

New style metadata

New in version 3.2.

The new standard way to specify project metadata is in a [project] table, as defined by PEP 621 <>. Flit works for now with either this or the older [tool.flit.metadata] table (described below), but it won't allow you to mix them.

A simple [project] table might look like this:

name = "astcheck"
authors = [

{name = "Thomas Kluyver", email = ""}, ] readme = "README.rst" classifiers = [
"License :: OSI Approved :: MIT License", ] requires-python = ">=3.5" dynamic = ["version", "description"]

The allowed fields are:

The name your package will have on PyPI. This field is required. For Flit, this also points to your package as an import name by default (see Module section if that needs to be different).
Version number as a string. If you want Flit to get this from a __version__ attribute, leave it out of the TOML config and include "version" in the dynamic field.
A one-line description of your project. If you want Flit to get this from the module docstring, leave it out of the TOML config and include "description" in the dynamic field.
A path (relative to the .toml file) to a file containing a longer description of your package to show on PyPI. This should be written in reStructuredText <>, Markdown or plain text, and the filename should have the appropriate extension (.rst, .md or .txt). Alternatively, readme can be a table with either a file key (a relative path) or a text key (literal text), and an optional content-type key (e.g. text/x-rst).
A version specifier for the versions of Python this requires, e.g. ~=3.3 or >=3.3,<4, which are equivalents.
A table with either a file key (a relative path to a license file) or a text key (the license text).
A list of tables with name and email keys (both optional) describing the authors of the project.
Same format as authors.
A list of words to help with searching for your package.
A list of Trove classifiers <>. Add Private :: Do Not Upload into the list to prevent a private package from being uploaded to PyPI by accident.
See Dependencies.
See URLs table.
See Scripts section.
See Entry points sections.
A list of field names which aren't specified here, for which Flit should find a value at build time. Only "version" and "description" are accepted.


The dependencies field is a list of other packages from PyPI that this package needs. Each package may be followed by a version specifier like >=4.1, and/or an environment marker <> after a semicolon. For example:

dependencies = [

"requests >=2.6",
"configparser; python_version == '2.7'", ]

The [project.optional-dependencies] table contains lists of packages needed for every optional feature. The requirements are specified in the same format as for dependencies. For example:

test = [

"pytest >=2.7.3",
"pytest-cov", ] doc = ["sphinx"]

You can call these optional features anything you want, although test and doc are common ones. You specify them for installation in square brackets after the package name or directory, e.g. pip install '.[test]'.

URLs table

Your project's page on <> can show a number of links. You can point people to documentation or a bug tracker, for example.

This section is called [project.urls] in the file. You can use any names inside it. Here it is for flit:

Documentation = ""
Source = ""

Scripts section

This section is called [project.scripts] in the file. Each key and value describes a shell command to be installed along with your package. These work like setuptools 'entry points'. Here's the section for flit:

flit = "flit:main"

This will create a flit command, which will call the function main() imported from flit.

A similar table called [project.gui-scripts] defines commands which launch a GUI. This only makes a difference on Windows, where GUI scripts are run without a console.

Entry points sections

You can declare entry points <> using sections named [project.entry-points.groupname]. E.g. to provide a pygments lexer from your package:

dogelang = "dogelang.lexer:DogeLexer"

In each package:name value, the part before the colon should be an importable module name, and the latter part should be the name of an object accessible within that module. The details of what object to expose depend on the application you're extending.

If the group name contains a dot, it must be quoted ("pygments.lexers" above). Script entry points are defined in scripts tables, so you can't use the group names console_scripts or gui_scripts here.

Module section

If your package will have different names for installation and import, you should specify the install (PyPI) name in the [project] table (see above), and the import name in a [tool.flit.module] table:

name = "pynsist"
# ...
name = "nsist"

Old style metadata

Flit's older way to specify metadata is in a [tool.flit.metadata] table, along with [tool.flit.scripts] and [tool.flit.entrypoints], described below. This is still recognised for now, but you can't mix it with New style metadata.

There are three required fields:

The name of the module/package, as you'd use in an import statement.
Your name
Your email address

e.g. for flit itself

module = "flit"
author = "Thomas Kluyver"
author-email = ""

Changed in version 1.1: home-page was previously required.

The remaining fields are optional:

A URL for the project, such as its Github repository.
A list of other packages from PyPI that this package needs. Each package may be followed by a version specifier like (>=4.1) or >=4.1, and/or an environment marker <> after a semicolon. For example:

requires = [

"requests >=2.6",
"configparser; python_version == '2.7'", ]

Lists of packages needed for every optional feature. The requirements are specified in the same format as for requires. The requirements of the two reserved extras test and doc as well as the extra dev are installed by flit install. For example:

test = [

"pytest >=2.7.3",
"pytest-cov", ] doc = ["sphinx"]

New in version 1.1.

A path (relative to the .toml file) to a file containing a longer description of your package to show on PyPI. This should be written in reStructuredText <>, Markdown or plain text, and the filename should have the appropriate extension (.rst, .md or .txt).
A list of Trove classifiers <>. Add Private :: Do Not Upload into the list to prevent a private package from uploading on PyPI by accident.
A version specifier for the versions of Python this requires, e.g. ~=3.3 or >=3.3,<4 which are equivalents.
If you want your package's name on PyPI to be different from the importable module name, set this to the PyPI name.
Comma separated list of words to help with searching for your package.
The name of a license, if you're using one for which there isn't a Trove classifier. It's recommended to use Trove classifiers instead of this in most cases.
Like author, for if you've taken over a project from someone else.

Here was the metadata section from flit using the older style:

author="Thomas Kluyver"

"flit_core >=2.2.0",
"tomli-w", ] requires-python=">=3.6" description-file="README.rst" classifiers=[
"Intended Audience :: Developers",
"License :: OSI Approved :: BSD License",
"Programming Language :: Python :: 3",
"Topic :: Software Development :: Libraries :: Python Modules", ]

URLs subsection

Your project's page on <> can show a number of links, in addition to the home-page URL described above. You can point people to documentation or a bug tracker, for example.

This section is called [tool.flit.metadata.urls] in the file. You can use any names inside it. Here it is for flit:

Documentation = ""

New in version 1.0.

Scripts section

A [tool.flit.scripts] table can be used along with [tool.flit.metadata]. It is in the same format as the newer [project.scripts] table described above.

Entry points sections

[tool.flit.entrypoints] tables can be used along with [tool.flit.metadata]. They are in the same format as the newer [project.entry-points] tables described above.

Sdist section

New in version 2.0.

When you use build_cmd or publish_cmd, Flit builds an sdist (source distribution) tarball containing the files that are checked into version control (git or mercurial). If you want more control, or it doesn't recognise your version control system, you can give lists of paths or glob patterns as include and exclude in this section. For example:

include = ["doc/"]
exclude = ["doc/*.html"]

These paths:

  • Always use / as a separator (POSIX style)
  • Must be relative paths from the directory containing pyproject.toml
  • Cannot go outside that directory (no ../ paths)
  • Cannot contain control characters or <>:"\\
  • Cannot use recursive glob patterns (**/)
  • Can refer to directories, in which case they include everything under the directory, including subdirectories
  • Should match the case of the files they refer to, as case-insensitive matching is platform dependent

Exclusions have priority over inclusions.

Flit command line interface

All operations use the flit command, followed by one of a number of subcommands.

Common options

Path to a config file specifying the module to build. The default is pyproject.toml.

Show the version of Flit in use.

Show help on the command-line interface.

Show more detailed logs about what flit is doing.

flit build

Build a wheel and an sdist (tarball) from the package.

Limit to building either wheel or sdist.

Generate a file in the sdist, so it can be installed by older versions of pip.

Don't generate a file in the sdist. This is the default. An sdist built without this will only work with tools that support PEP 517, but the wheel will still be usable by any compatible tool.

Changed in version 3.5: Generating disabled by default.

flit publish

Build a wheel and an sdist (tarball) from the package, and upload them to PyPI or another repository.

Limit to publishing either wheel or sdist. You should normally publish the two formats together.

Generate a file in the sdist, so it can be installed by older versions of pip.

Don't generate a file in the sdist. This is the default. An sdist built without this will only work with tools that support PEP 517, but the wheel will still be usable by any compatible tool.

Changed in version 3.5: Generating disabled by default.

Name of a repository to upload packages to. Should match a section in ~/.pypirc. The default is pypi.

The .pypirc config file to be used. The default is ~/.pypirc.



flit install

Install the package on your system.

By default, the package is installed to the same Python environment that Flit itself is installed in; use --python or FLIT_INSTALL_PYTHON to override this.

If you don't have permission to modify the environment (e.g. the system Python on Linux), Flit may do a user install instead. Use the --user or --env flags to force this one way or the other, rather than letting Flit guess.

Symlink the module into site-packages rather than copying it, so that you can test changes without reinstalling the module.

Create a .pth file in site-packages rather than copying the module, so you can test changes without reinstalling. This is a less elegant alternative to --symlink, but it works on Windows, which typically doesn't allow symlinks.

Which dependencies to install. One of all, production, develop, or none. all and develop install the extras test, doc, and dev. Default all.

Which named extra features to install dependencies for. Specify all to install all optional dependencies, or a comma-separated list of extras. Default depends on --deps.

Do a user-local installation. This is the default if flit is not in a virtualenv or conda env (if the environment's library directory is read-only and site.ENABLE_USER_SITE is true).

Install into the environment - the opposite of --user. This is the default in a virtualenv or conda env (if the environment's library directory is writable or site.ENABLE_USER_SITE is false).

Install for another Python, identified by the path of the python executable. Using this option, you can install a module for Python 2, for instance. See FLIT_INSTALL_PYTHON if this option is not given.

Changed in version 2.1: Added FLIT_INSTALL_PYTHON and use its value over the Python running Flit when an explicit --python option is not given.


Flit calls pip to do the installation. You can set any of pip's options using its environment variables <>.

When you use the --symlink or --pth-file options, pip is used to install dependencies. Otherwise, Flit builds a wheel and then calls pip to install that.

flit init

Create a new pyproject.toml config file by prompting for information about the module in the current directory.

Environment variables

New in version 0.10.

Setting this to any non-empty value will stop flit from making network connections (unless you explicitly ask to upload a package). This is intended for downstream packagers, so if you use this, it's up to you to ensure any necessary dependencies are installed.

By default, flit install will fail when run as root on POSIX systems, because installing Python modules systemwide is not recommended. Setting this to any non-empty value allows installation as root. It has no effect on Windows.

New in version 0.11.

Set a username, password, and index URL for uploading packages. See uploading packages with environment variables for more information.

New in version 0.13.

Setting this to any non-empty value tells Flit to continue if it detects invalid metadata, instead of failing with an error. Problems will still be reported in the logs, but won't cause Flit to stop.

If the metadata is invalid, uploading the package to PyPI may fail. This environment variable provides an escape hatch in case Flit incorrectly rejects your valid metadata. If you need to use it and you believe your metadata is valid, please open an issue <>.

New in version 2.1.

Set a default Python interpreter for flit install to use when --python is not specified. The value can be either an absolute path, or a command name (which will be found in PATH). If this is unset or empty, the module is installed for the copy of Python that is running Flit.

To make reproducible builds, set this to a timestamp as a number of seconds since the start of the year 1970 in UTC, and document the value you used. On Unix systems, you can get a value for the current time by running:

date +%s


The SOURCE_DATE_EPOCH specification <>

Controlling package uploads

The command flit publish will upload your package to a package index server. The default settings let you upload to PyPI <>, the default Python Package Index, with a single user account.

If you want to upload to other servers, or with more than one user account, or upload packages from a continuous integration job, you can configure Flit in two main ways:

Using .pypirc

You can create or edit a config file in your home directory, ~/.pypirc that will be used by default or you can specify a custom location. This is also used by other Python tools such as twine <>.

For instance, to upload a package to the Test PyPI server <> instead of the normal PyPI, use a config file looking like this:

index-servers =

testpypi [pypi] repository = username = sirrobin # Replace with your PyPI username [testpypi] repository = username = sirrobin # Replace with your TestPyPI username

You can select an index server from this config file with the --repository option:

flit publish --repository testpypi

If you don't use this option, Flit will use the server called pypi in the config file. If that doesn't exist, it uploads to PyPI at by default.

If you publish a package and you don't have a .pypirc file, Flit will create it to store your username.

Flit tries to store your password securely using the keyring <> library. If keyring is not installed, Flit will ask for your password for each upload. Alternatively, you can also manually add your password to the .pypirc file (password = ...)

Using environment variables

You can specify a server to upload to with FLIT_INDEX_URL, and pass credentials with FLIT_USERNAME and FLIT_PASSWORD. Environment variables take precedence over the config file, except if you use the --repository option to explicitly pick a server from the config file.

This can make it easier to automate uploads, for example to release packages from a continuous integration job.


Storing a password in an environment variable is convenient, but it's easy to accidentally leak it <>. Look out for scripts that helpfully print all environment variables for debugging, and remember that other scripts and libraries you run in that environment have access to your password.

Reproducible builds

New in version 0.8.

Wheels built by flit are reproducible: if you build from the same source code, you should be able to make wheels that are exactly identical, byte for byte. This is useful for verifying software. For more details, see <>.

There is a caveat, however: wheels (which are zip files) include the modification timestamp from each file. This will probably be different on each computer, because it indicates when your local copy of the file was written, not when it was changed in version control. These timestamps can be overridden by the environment variable SOURCE_DATE_EPOCH.

flit publish
# Record the value of SOURCE_DATE_EPOCH in release notes for reproduction

Changed in version 0.12: Normalising permission bits

Flit normalises the permission bits of files copied into a wheel to either 755 (executable) or 644. This means that a file is readable by all users and writable only by the user who owns it.

The most popular version control systems only track the executable bit, so checking out the same repository on systems with different umasks (e.g. Debian and Fedora) produces files with different permissions. With Flit 0.11 and earlier, this difference would produce non-identical wheels.

Why use Flit?

Make the easy things easy and the hard things possible is an old motto from the Perl community. Flit is entirely focused on the easy things part of that, and leaves the hard things up to other tools.

Specifically, the easy things are pure Python packages with no build steps (neither compiling C code, nor bundling Javascript, etc.). The vast majority of packages on PyPI are like this: plain Python code, with maybe some static data files like icons included.

It's easy to underestimate the challenges involved in distributing and installing code, because it seems like you just need to copy some files into the right place. There's a whole lot of metadata and tooling that has to work together around that fundamental step. But with the right tooling, a developer who wants to release their code doesn't need to know about most of that.

What, specifically, does Flit make easy?

  • flit init helps you set up the information Flit needs about your package.
  • Subpackages are automatically included: you only need to specify the top-level package.
  • Data files within a package directory are automatically included. Missing data files has been a common packaging mistake with other tools.
  • The version number is taken from your package's __version__ attribute, so that always matches the version that tools like pip see.
  • flit publish uploads a package to PyPI, so you don't need a separate tool to do this.

Setuptools, the most common tool for Python packaging, now has shortcuts for many of the same things. But it has to stay compatible with projects published many years ago, which limits what it can do by default.

Flit also has some support for reproducible builds, a feature which some people care about.

There have been many other efforts to improve the user experience of Python packaging, such as pbr <>, but before Flit, these tended to build on setuptools and distutils. That was a pragmatic decision, but it's hard to build something radically different on top of those libraries. The existence of Flit spurred the development of new standards, like PEP 518 <> and PEP 517 <>, which are now used by other packaging tools such as Poetry <> and Enscons <>.

Other options

If your package needs a build step, you won't be able to use Flit. Setuptools <> is the de-facto standard, but newer tools such as Enscons <> also cover this case.

Flit also doesn't help you manage dependencies: you have to add them to pyproject.toml by hand. Tools like Poetry <> and Pipenv <> have features which help add and update dependencies on other packages.


Flit is itself packaged using Flit, as are some foundational packaging tools such as pep517. So where can you start if you need to install everything from source?


For most users, pip handles all this automatically. You should only need to deal with this if you're building things entirely from scratch, such as putting Python packages into another package format.

The key piece is flit_core. This is a package which can build itself using nothing except Python and the standard library. From an unpacked source archive, you can run python, of which the crucial part is:

from flit_core import buildapi
whl_fname = buildapi.build_wheel('dist/')
print(os.path.join('dist', whl_fname))

This produces a .whl wheel file, which you can unzip into your site-packages folder (or equivalent) to make flit_core available for building other packages. (You could also just copy flit_core from the source directory, but without the .dist-info folder, tools like pip won't know that it's installed.)

As of version 3.6, flit_core bundles the tomli TOML parser, to avoid a dependency cycle. If you need to unbundle it, you will need to special-case installing flit_core and/or tomli to get around that cycle.

I recommend that you get the build <> and installer <> packages (and their dependencies) installed as the goal of the bootstrapping phase. These tools together can be used to install any other Python packages: build to create wheels and installer to install them.

Developing Flit

To get a development installation of Flit itself:

git clone
cd flit
python3 -m pip install docutils requests toml

This links Flit into the current Python environment, so you can make changes and try them without having to reinstall each time.


To run the tests in separate environments for each available Python version:


tox <> has many options.

To run the tests in your current environment, run:


Release history

Version 3.6

flit_core now bundles the tomli <> TOML parser library (version 1.2.3) to avoid a circular dependency between flit_core and tomli (
). This means flit_core now has no dependencies except Python itself, both at build time and at runtime, simplifying bootstrapping.

Version 3.5.1

Fix development installs with flit install --symlink and --pth-file, which were broken in 3.5.0, especially for packages using a src folder (

Version 3.5

  • You can now use Flit to distribute a module or package inside a namespace package (as defined by PEP 420 <>). To do this, specify the import name of the concrete, inner module you are packaging - e.g. name = "" - either in the [project] table, or under [tool.flit.module] if you want to use a different name on PyPI (
  • Flit no longer generates a file in sdists (.tar.gz packages) by default (
    ). Modern packaging tools don't need this. You can use the --setup-py flag to keep adding it for now, but this will probably be removed at some point in the future.
  • Fixed how flit init handles authors' names with non-ASCII characters (
  • When flit init generates a LICENSE file, the new pyproject.toml now references it (

Version 3.4

  • Python 3.6 or above is now required, both for flit and flit_core.
  • Add a --setup-py option to flit build and flit publish, and a warning when neither this nor --no-setup-py are specified (
    ). A future version will stop generating files in sdists by default.
  • Add support for standardised editable installs - pip install -e - according to PEP 660 <> (
  • Add a --pypirc option for flit publish to specify an alternative path to a .pypirc config file describing package indexes (
  • Fix installing dependencies specified in a [project] table (
  • Fix building wheels when SOURCE_DATE_EPOCH (see reproducible) is set to a date before 1980 (
  • Switch to using the tomli <> TOML parser, in common with other packaging projects (
    ). This supports TOML version 1.0.
  • Add a document on bootstrap (

Version 3.3

  • PKG-INFO files in sdists are now generated the same way as METADATA in wheels, fixing some issues with sdists (
  • flit publish now sends SHA-256 hashes, fixing uploads to GitLab package repositories (
  • The [project] metadata table from PEP 621 <> is now fully supported and documented. Projects using this can now specify requires = ["flit_core >=3.2,<4"] in the [build-system] table.

Version 3.2

  • Experimental support for specifying metadata in a [project] table in pyproject.toml as specified by PEP 621 <> (
    ). If you try using this, please specify requires = ["flit_core >=3.2.0,<3.3"] in the [build-system] table for now, in case it needs to change for the next release.
  • Fix writing METADATA file with multi-line information in certain fields such as Author (
  • Fix building wheel when a directory such as LICENSES appears in the project root directory (

Version 3.1

  • Update handling of names & version numbers in wheel filenames and .dist-info folders in line with changes in the specs (
  • Switch from the deprecated pytoml package to toml (
  • Fix specifying backend-path in pyproject.toml for flit-core (as a list instead of a string).

Version 3.0

Breaking changes:

  • Projects must now provide Flit with information in pyproject.toml files, not the older flit.ini format (
  • flit_core once again requires Python 3 (>=3.4). Packages that support Python 2 can still be built by flit_core 2.x, but can't rely on new features (
  • The deprecated flit installfrom command was removed (
    ). You can use pip install git+ instead.

Features and fixes:

  • Fix building sdists from a git repository with non-ASCII characters in filenames (
  • Fix identifying the version number when the code contains a subscript assignment before __version__ = (
  • Script entry points can now use a class method (
  • Set suitable permission bits on metadata files in wheels (
  • Fixed line endings in the RECORD file when installing on Windows (
  • Support for recording the source of local installations, as in PEP 610 <> (
  • flit init will check for a README in the root of the project and automatically set it as description-file (
  • Pygments is not required for checking reStructuredText READMEs (
  • Packages where the version number can be recognised without executing their code don't need their dependencies installed to build, which should make them build faster (
  • Ensure the installed RECORD file is predictably ordered (

Version 2.3

  • New projects created with init_cmd now declare that they require flit_core >=2,<4 (
    ). Any projects using pyproject.toml (not flit.ini) should be compatible with flit 3.x.
  • Fix selecting files from a git submodule to include in an sdist (
  • Fix checking classifiers when no writeable cache directory is available (
  • Better errors when trying to install to a mis-spelled or missing Python interpreter (
  • Fix specifying --repository before upload (
    ). Passing the option like this is deprecated, and you should now pass it after upload.
  • Documentation improvements (

Version 2.2

  • Allow underscores in package names with Python 2 (
  • Add a --no-setup-py option to build sdists without a backwards-compatible file (
  • Fix the generated file for packages using a src/ layout (
  • Fix detecting when more than one file matches the module name specified (
  • Fix installing to a venv on Windows with the --python option (
  • Don't echo the command in scripts installed with --symlink or --pth-file on Windows (
  • New script to set up a development installation of Flit from the repository (

Version 2.1

  • Use compression when adding files to wheels.
  • Added the FLIT_INSTALL_PYTHON environment variable (
    ), to configure flit to always install into a Python other than the one it's running on.
  • flit_core uses the intreehooks shim package to load its bootstrapping backend, until a released version of pip supports the standard backend-path mechanism.

Version 2.0

Flit 2 is a major architecture change. The flit_core package now provides a PEP 517 <> backend for building packages, while flit is a command line interface extending that.

The build backend works on Python 2, so tools like pip should be able to install packages built with flit from source on Python 2. The flit command requires Python 3.5 or above. You will need to change the build-system table in your pyproject.toml file to look like this:

requires = ["flit_core >=2,<4"]
build-backend = "flit_core.buildapi"

Other changes include:

  • Support for storing your code under a src/ folder (
    ). You don't need to change any configuration if you do this.
  • Options to control what files are included in an sdist - see pyproject_toml_sdist for the details.
  • Requirements can specify a URL 'direct reference', as an alternative to a version number, with the syntax defined in PEP 440 <>: requests @
  • Fix the shebang of scripts installed with the --python option and the --symlink flag (
  • Installing with --deps develop now installs normal dependencies as well as development dependencies.
  • Author email is no longer required in the metadata table (
  • More error messages are now shown without a traceback (

Version 1.3

  • Fix for building sdists from a subdirectory in a Mercurial repository (
  • Fix for getting the docstring and version from modules defining their encoding (
  • Fix for installing packages with flit installfrom (
  • Packages with requirements no longer get a spurious Provides-Extra: .none metadata entry (
  • Better check of whether python-requires includes any Python 2 version (
  • Better check of home page URLs in flit init (
  • Better error message when the description file is not found (
  • Updated a help message to refer to pyproject.toml (
  • Improve tests of flit init (

Version 1.2.1

  • Fix for installing packages with flit install.
  • Make requests_download an extra dependency, to avoid a circular build dependency. To use flit installfrom, you can install with pip install flit[installfrom]. Note that the installfrom subcommand is deprecated, as it will soon be possible to use pip to install Flit projects directly from a VCS URL.

Version 1.2

  • Fixes for packages specifying requires-extra: sdists should now work, and environment markers can be used together with requires-extra.
  • Fix running flit installfrom without a config file present in the working directory.
  • The error message for a missing or empty docstring tells you what file the docstring should be in.
  • Improvements to documentation on version selectors for requirements.

Version 1.1

  • Packages can now have 'extras', specified as requires-extra in the pyproject.toml file. These are additional dependencies for optional features.
  • The home-page metadata field is no longer required.
  • Additional project URLs are now validated.
  • flit -V is now equivalent to flit --version.
  • Various improvements to documentation.

Version 1.0

  • The description file may now be written in reStructuredText, Markdown or plain text. The file extension should indicate which of these formats it is (.rst, .md or .txt). Previously, only reStructuredText was officially supported.
  • Multiple links (e.g. documentation, bug tracker) can now be specified in a new [tool.flit.metadata.urls] section of pyproject.toml.
  • Dependencies are now correctly installed to the target Python when you use the --symlink or --pth-file options.
  • Dependencies are only installed to the Python where Flit is running if it fails to get the docstring and version number without them.
  • The commands deprecated in 0.13—flit wheel, flit sdist and flit register—have been removed.

Although version 1.0 sounds like a milestone, there's nothing that makes this release especially significant. It doesn't represent a step change in stability or completeness. Flit has been gradually maturing for some time, and I chose this point to end the series of 0.x version numbers.

Version 0.13

  • Better validation of several metadata fields (dist-name, requires, requires-python, home-page), and of the version number.
  • New FLIT_ALLOW_INVALID environment variable to ignore validation failures in case they go wrong.
  • The list of valid classifiers is now fetched from Warehouse (, rather than the older site.
  • Deprecated flit wheel and flit sdist subcommands: use build_cmd.
  • Deprecated flit register: you can no longer register a package separately from uploading it.

Version 0.12.3

  • Fix building and installing packages with a - in the distribution name.
  • Fix numbering in README.

Version 0.12.2

New tool to convert flit.ini to pyproject.toml:

python3 -m flit.tomlify

Use the PAX tar format for sdists, as specified by PEP 517.

Version 0.12.1

  • Restore dependency on zipfile36 backport package.
  • Add some missing options to documentation of flit install subcommand.
  • Rearrange environment variables in the docs.

Version 0.12

  • Switch the config to pyproject.toml by default instead of flit.ini, and implement the PEP 517 API.
  • A new option --pth-file allows for development installation on Windows (where --symlink usually won't work).
  • Normalise file permissions in the zip file, making builds more reproducible across different systems.
  • Sdists (.tar.gz packages) can now also be reproducibly built by setting SOURCE_DATE_EPOCH.
  • For most modules, Flit can now extract the version number and docstring without importing it. It will still fall back to importing where getting these from the AST fails.
  • flit build will build the wheel from the sdist, helping to ensure that files aren't left out of the sdist.
  • All list fields in the INI file now ignore blank lines (requires, dev-requires, classifiers).
  • Fix the path separator in the RECORD file of a wheel built on Windows.
  • Some minor fixes to building reproducible wheels.
  • If building a wheel fails, the temporary file created will be cleaned up.
  • Various improvements to docs and README.

Version 0.11.4

  • Explicitly open various files as UTF-8, rather than relying on locale encoding.
  • Link to docs from README.
  • Better test coverage, and a few minor fixes for problems revealed by tests.

Version 0.11.3

Fixed a bug causing failed uploads when the password is entered in the terminal.

Version 0.11.2

A couple of behaviour changes when uploading to warehouse.

Version 0.11.1

  • Fixed a bug when you use flit to build an sdist from a subdirectory inside a VCS checkout. The VCS is now correctly detected.
  • Fix the rst checker for newer versions of docutils, by upgrading the bundled copy of readme_renderer.

Version 0.11

Flit can now build sdists (tarballs) and upload them to PyPI, if your code is in a git or mercurial repository. There are new commands:
  • flit build builds both a wheel and an sdist.
  • flit publish builds and uploads a wheel and an sdist.

Smarter ways of getting the information needed for upload:
  • If you have the keyring <> package installed, flit can use it to store your password, rather than keeping it in plain text in ~/.pypirc.
  • If ~/.pypirc does not already exist, and you are prompted for your username, flit will write it into that file.
  • You can provide the information as environment variables: FLIT_USERNAME, FLIT_PASSWORD and FLIT_INDEX_URL. Use this to upload packages from a CI service, for instance.

  • Include 'LICENSE' or 'COPYING' files in wheels.
  • Fix for flit install --symlink inside a virtualenv.

Version 0.10

Downstream packagers can use the FLIT_NO_NETWORK environment variable to stop flit downloading data from the network.

Version 0.9

  • flit install and flit installfrom now take an optional --python argument, with the path to the Python executable you want to install it for. Using this, you can install modules to Python 2.
  • Installing a module normally (without --symlink) builds a wheel and uses pip to install it, which should work better in some corner cases.

Version 0.8

  • A new flit installfrom subcommand to install a project from a source archive, such as from Github.
  • Reproducible builds - you can produce byte-for-byte identical wheels.
  • A warning for non-canonical version numbers according to PEP 440 <>.
  • Fix for installing projects on Windows.
  • Better error message when module docstring is only whitespace.

Version 0.7

  • A new dev-requires field in the config file for development requirements, used when doing flit install.
  • Added a --deps option for flit install to control which dependencies are installed.
  • Flit can now be invoked with python -m flit.

Version 0.6

  • flit install now ensures requirements specified in flit.ini are installed, using pip.
  • If you specify a description file, flit now warns you if it's not valid reStructuredText (since invalid reStructuredText is treated as plain text on PyPI).
  • Improved the error message for mis-spelled keys in flit.ini.

Version 0.5

  • A new flit init command to quickly define the essential basic metadata for a package.
  • Support for entry points.
  • A new flit register command to register a package without uploading it, for when you want to claim a name before you're ready to release.
  • Added a --repository option for specifying an alternative PyPI instance.
  • Added a --debug flag to show debug-level log messages.
  • Better error messages when the module docstring or __version__ is missing.

Version 0.4

  • Users can now specify dist-name in the config file if they need to use different names on PyPI and for imports.
  • Classifiers are now checked against a locally cached list of valid classifiers.
  • Packages can be locally installed into environments for development.
  • Local installation now creates a PEP 376 .dist-info folder instead of .egg-info.

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Thomas Kluyver


2021, Thomas Kluyver

December 30, 2021 3.6.0