basedpyright 1.5.0

static type checking for Python (but based)

basedpyright

Basedpyright is a static type checker for Python that is built on top of the work done by the pyright project.

why?

the main motivation behind this fork was the fact that pyright has several serious issues that the maintainers didn't want to address, and many bugs that they consider to be intentional behavior. here is a list of some of its major problems that we've resolved in basedpyright:

ability to pin the version used by vscode

in pyright, if the vscode extension gets updated, you may see errors in your project that don't appear in the CI, or vice-versa. see this issue.

basedpyright fixes this problem by adding an importStrategy option to the extension, which defaults to looking in your project for the basedpyright pypi package.

published as a pypi package - no nodejs required

pyright is only published as an npm package, which requires you to install nodejs. the version on pypi is just an unofficial wrapper that installs node and the npm package the first time you invoke the cli, which is quite flaky.

python developers should not be expected to have to install nodejs in order to typecheck their python code. it should just be a regular pypi package like mypy, ruff, and pretty much all other python tooling. this is why basedpyright is officially published on pypi, which comes bundled with the npm package.

reporting errors on unreachable code

pyright often incorrectly marks code as unreachable. in most cases, unreachable code is a mistake and therefore should be an error, but pyright does not have an option to report unreachable code. in fact, unreachable code is not even type-checked at all:

if sys.platform == "win32": 
  1 + "" # no error

by default, pyright will treat the body in the code above as unreachable if pyright itself was run on an operating system other than windows. this is bad of course, because chances are if you write such a check, you intend for your code to be executed on multiple platforms.

to make things worse, unreachable code is not even type-checked, so the obviously invalid 1 + "" above will go completely unnoticed by the type checker.

basedpyright solves this issue with a reportUnreachable option, which will report an error on such unchecked code. in this example, you can update your pyright config to specify more platforms using the pythonPlatform option if you intend for the code to be reachable.

errors on invalid configuration

in pyright, if you have any invalid config, it may or may not print a warning to the console, then it will continue type-checking and the exit code will be 0 as long as there were no type errors:

[tool.pyright]
mode = "strict"  # wrong! the setting you're looking for is called `typeCheckingMode`

in this example, it's very easy for errors to go undetected because you thought you were on strict mode, but in reality pyright just ignored the setting and silently continued type-checking on "basic" mode.

to solve this problem, basedpyright will exit with code 3 on any invalid config.

option to fully ban the Any type

pyright has a few options to ban "Unknown" types such as reportUnknownVariableType, reportUnknownParameterType, etc. but "Unknown" is not a real type, rather a distinction pyright uses used to represent Anys that come from untyped code or unfollowed imports. if you want to ban all kinds of Any, pyright has no way to do that:

def foo(bar, baz: Any) -> Any:
    print(bar) # error: unknown type
    print(baz) # no error

basedpyright introduces the reportAny option, which will report an error on usages of anything typed as Any.

re-implementing pylance-exclusive features

basedpyright re-implements some of the features that microsoft made exclusive to their closed-source pylance extension. for more information about the differences between pyright and pylance, see here

import suggestion code actions

pyright only supports import suggestions as autocomplete suggestions, but not as quick fixes (see this issue).

basedpyright re-implements pylance's import suggestion code actions:

semantic highlighting

before	after

basedpyright re-implements pylance's semantic highlighting and improves upon it in some cases (eg. variables marked as Final have the correct "read-only" colour)

initial implementation of the semantic highlighting provider was adapted from the pyright-inlay-hints project.

inlay hints

Initial implementations of the inlay hint provider were adapted from coc-pyright and pyright-inlay-hints.

basedpyright integrates it into the vscode extension:

reporting errors on invalid "relative" imports

pyright allows invalid imports such as this:

# ./module_name/foo.py:

# ./module_name/bar.py:
import foo # wrong! should be `import module_name.foo` or `from module_name import foo`

this may look correct at first glance, and will work when running bar.py directly as a script, but when it's imported as a module, it will crash:

# ./main.py:
import module_name.bar  # ModuleNotFoundError: No module named 'foo' 

basedpyright bans imports like this. if you want to do a relative import, the correct way to do it is by prefixing the module name with a .:

# ./module_name/bar.py:
import .foo

fixes for the reportRedeclaration and reportDuplicateImport rules

pyright does not report redeclarations if the redeclaration has the same type:

foo: int = 1
foo: int = 2  # no error

nor does it care if you have a duplicated import in multiple different import statements, or in aliases:

from foo import bar
from bar import bar  # no error
from baz import foo as baz, bar as baz  # no error

pyright solves both of these problems by always reporting an error on a redeclaration or an import with the same name as an existing import.

better defaults

we believe that type checkers and linters should be as strict as possible by default, making the user aware of all the available rules so they can more easily make informed decisions about which rules they don't want enabled in their project. that's why the following defaults have been changed in basedpyright

typeCheckingMode

used to be basic, but now defaults to all. in the future we intend to add baseline to allow for easy adoption of more strict rules in existing codebases.

pythonPlatform

used to assume that the operating system pyright is being run on is the only operating system your code will run on, which is rarely the case. in basedpyright, pythonPlatform defaults to All, which assumes your code can run on any operating system.

basedmypy feature parity

basedmypy is a fork of mypy with a similar goal in mind: to fix some of the serious problems in mypy that do not seem to be a priority for the maintainers. it also adds many new features which may not be standardized but greatly improve the developer experience when working with python's far-from-perfect type system.

we aim to port most of basedmypy's features to basedpyright, however as mentioned above our priority is to first fix the critical problems with pyright.

pypi package

basedpyright differs from pyright by publishing the command line tool as a pypi package instead of an npm package. this makes it far more convenient for python developers to use.

> basedpyright --help
Usage: basedpyright [options] files...
  Options:
  --createstub <IMPORT>              Create type stub file(s) for import
  --dependencies                     Emit import dependency information
  -h,--help                          Show this help message
  --ignoreexternal                   Ignore external imports for --verifytypes
  --level <LEVEL>                    Minimum diagnostic level (error or warning)
  --outputjson                       Output results in JSON format
  -p,--project <FILE OR DIRECTORY>   Use the configuration file at this location
  --pythonplatform <PLATFORM>        Analyze for a specific platform (Darwin, Linux, Windows)
  --pythonpath <FILE>                Path to the Python interpreter
  --pythonversion <VERSION>          Analyze for a specific version (3.3, 3.4, etc.)
  --skipunannotated                  Skip analysis of functions with no type annotations
  --stats                            Print detailed performance stats
  -t,--typeshedpath <DIRECTORY>      Use typeshed type stubs at this location
  -v,--venvpath <DIRECTORY>          Directory that contains virtual environments
  --verbose                          Emit verbose diagnostics
  --verifytypes <PACKAGE>            Verify type completeness of a py.typed package
  --version                          Print Pyright version and exit
  --warnings                         Use exit code of 1 if warnings are reported
  -w,--watch                         Continue to run and watch for changes
  -                                  Read files from stdin

vscode extension

install

install the extension from the vscode extension marketplace

usage

the basedpyright vscode extension will automatically look for the pypi package in your python environment. see the recommended setup section below for more information

pylance vs basedpyright

pylance is microsoft's closed-source vscode extension built on top of the pyright language server with some additional exclusive functionality (see the pylance FAQ for more information). normally when the pylance extension is enabled, the pyright extension will disable itself to avoid conflicting with it. unfortunately since it's closed-source, there's no way for us to update it to use basedpyright instead. so we intend to re-implement its exclusive features in basedpyright.

if you don't depend on any pylance-exclusive features, the recommended solution is to disable/uninstall the pylance extension.

if you do want to continue using pylance, all of the options and commands in basedpyright have been renamed to avoid any conflicts with the pylance extension, and the restriction that prevents both extensions from being enabled at the same time has been removed. for an optimal experience you should disable pylance's type checking and disable basedpyright's language server features. see the recommended setup section below for details.

recommended setup

it's recommended to use both the basedpyright cli and vscode extension in your project. the vscode extension is for local development and the cli is for your CI.

below are the changes i recommend making to your project when adopting basedpyright

.vscode/extensions.json

{
  "recommendations": [
    "detachhead.basedpyright" // this will prompt developers working on your project to install the extension
  ],
  "unwantedRecommendations": [
    "ms-python.vscode-pylance" // if not using pylance (see below)
  ]
}

.vscode/settings.json

if not using pylance

• remove any settings starting with python.analysis, as they are not used by basedpyright. you should instead set these settings using the tool.basedpyright (or tool.pyright) section in pyroject.toml (see below)
• disable the built in language server support from the python extension, as it seems to conflict with basedpyright's language server:

  {
      "python.languageServer": "None"
  }
  

if using pylance

• disable pylance's type-checking by setting "python.analysis.typeCheckingMode" to "off". this will prevent pylance from displaying duplicated errors from its bundled pyright version alongside the errors already displayed by the basedpyright extension.
• disable basedpyright's LSP features by setting "basedpyright.disableLanguageServices" to true. this will prevent duplicated hover text and other potential issues with pylance's LSP. keep in mind that this may result in some inconsistent behavior since pylance uses its own version of the pyright LSP.

{
    "python.analysis.typeCheckingMode": "off",
    "basedpyright.disableLanguageServices": true
}

.github/workflows/check.yaml

jobs:
  check:
    steps:
      - run: ...  # checkout repo, install dependencies, etc
      - run: basedpyright  # add this line

pyproject.toml

we recommend using pdm or poetry to manage your dependencies.

[tool.pdm.dev-dependencies]  # or the poetry equivalent
dev = [
    "basedpyright", # you can pin the version here if you want, or just rely on the lockfile
]

[tool.basedpyright]
# many settings are not enabled even in strict mode, which is why basedpyright includes an "all" option
# you can then decide which rules you want to disable
typeCheckingMode = "all"

pinning your dependencies is important because it allows your CI builds to be reproducible (ie. two runs on the same commit will always produce the same result). basedpyright ensures that the version of pyright used by vscode always matches this pinned version