Refurb

A tool for refurbishing and modernizing Python codebases.

Example

# main.py

for filename in ["file1.txt", "file2.txt"]:
    with open(filename) as f:
        contents = f.read()

    lines = contents.splitlines()

    for line in lines:
        if not line or line.startswith("# ") or line.startswith("// "):
            continue

        for word in line.split():
            print(f"[{word}]", end="")

        print("")

Running:

$ refurb main.py
main.py:3:17 [FURB109]: Use `in (x, y, z)` instead of `in [x, y, z]`
main.py:4:5 [FURB101]: Use `y = Path(x).read_text()` instead of `with open(x, ...) as f: y = f.read()`
main.py:10:40 [FURB102]: Replace `x.startswith(y) or x.startswith(z)` with `x.startswith((y, z))`
main.py:16:9 [FURB105]: Use `print() instead of `print("")`

Installing

Before installing, it is recommended that you setup a virtual environment.

$ pip3 install refurb
$ refurb file.py folder/

Note: Refurb only supports Python 3.10. It can check Python 3.6 code and up, but Refurb itself must be ran through Python 3.10.

Explanations For Checks

You can use refurb --explain FURB123, where FURB123 is the error code you are trying to look up. For example:

$ refurb --explain FURB123
Don't cast a variable or literal if it is already of that type. For
example:

Bad:

```
name = str("bob")
num = int(123)
```

Good:

```
name = "bob"
num = 123
```

Ignoring Errors

Use --ignore 123 to ignore error 123. The error code can be in the form FURB123 or 123. This flag can be repeated.

The FURB prefix indicates that this is a built-in error. The FURB prefix is optional, but for all other errors (ie, ABC123), the prefix is required.

You can also use inline comments to disable errors:

x = int(0)  # noqa: FURB123
y = list()  # noqa

Here, noqa: FURB123 specifically ignores the FURB123 error for that line, and noqa ignores all errors on that line.

Enabling/Disabling Checks

Certain checks are disabled by default, and need to be enabled first. You can do this using the --enable ERR flag, where ERR is the error code of the check you want to enable. A disabled check differs from an ignored check in that a disabled check will never be loaded, whereas an ignored check will be loaded, an error will be emitted, and the error will be suppressed.

The opposite of --enable is --disable, which will disable a check. When --enable and --disable are both specified via the command line, whichever one comes last will take precedence. When using enable and disable via the config file, disable will always take precedence.

Use the --disable-all flag to disable all checks. This allows you to incrementally --enable checks as you see fit, as opposed to adding a bunch of --ignore flags. To use this in the config file, set disable_all to true.

Use the --enable-all flag to enable all checks by default. This allows you to opt into all checks that Refurb (and Refurb plugins) have to offer. This is a good option for new codebases. To use this in a config file, set enable_all to true.

In the config file, disable_all/enable_all is applied first, and then the enable and disable fields are applied afterwards.

Note that disable_all and enable_all are mutually exclusive, both on the command line and in the config file. You will get an error if you try to specify both.

You can also disable checks by category using the #category syntax. For example, --disable "#readability" will disable all checks with the readability category. The same applies for enable and ignore. Also, if you disable an entire category you can still explicitly re-enable a check in that category.

Note that #readability is wrapped in quotes because your shell will interpret the # as the start of a comment.

Setting Python Version

Use the --python-version flag to tell Refurb which version of Python your codebase is using. This should allow for better detection of language features, and allow for better error messages. The argument for this flag must be in the form x.y, for example, 3.10.

Overriding Mypy Flags

This is typically used for development purposes, but can also be used to better fine-tune Mypy from within Refurb. Any command line arguments after -- are passed to Mypy. For example:

$ refurb files -- --show-traceback

This tells Mypy to show a traceback if it crashes.

You can also use this in the config file by assigning an array of values to the mypy_args field. Note that any Mypy arguments passed via the command line arguments will override the mypy_args field in the config file.

Configuring Refurb

In addition to the command line arguments, you can also add your settings in the pyproject.toml file. For example, the following command line arguments:

refurb file.py --ignore 100 --load some_module --quiet

Corresponds to the following in your pyproject.toml file:

[tool.refurb]
ignore = [100]
load = ["some_module"]
quiet = true

Now all you need to type is refurb file.py!

Note that the values in the config file will be merged with the values specified via the command line. In the case of boolean arguments like --quiet, the command line arguments take precedence. All other arguments (such as ignore and load) will be combined.

You can use the --config-file flag to tell Refurb to use a different config file from the default pyproject.toml file. Note that it still must be in the same form as the normal pyproject.toml file.

Ignore Checks Per File/Folder

If you have a large codebase you might want to ignore errors for certain files or folders, which allows you to incrementally fix errors as you see fit. To do that, add the following to your pyproject.toml file:

# these settings will be applied globally
[tool.refurb]
enable_all = true

# these will only be applied to the "src" folder
[[tool.refurb.amend]]
path = "src"
ignore = ["FURB123", "FURB120"]

# these will only be applied to the "src/util.py" file
[[tool.refurb.amend]]
path = "src/util.py"
ignore = ["FURB125", "FURB148"]

Note that only the ignore field is available in the amend sections. This is because a check can only be enabled/disabled for the entire codebase, and cannot be selectively enabled/disabled on a per-file basis. Assuming a check is enabled though, you can simply ignore the errors for the files of your choosing.

Using Refurb With pre-commit

You can use Refurb with pre-commit by adding the following to your .pre-commit-config.yaml file:

  - repo: https://github.com/dosisod/refurb
    rev: REVISION
    hooks:
      - id: refurb

Replacing REVISION with a version or SHA of your choosing (or leave it blank to let pre-commit find the most recent one for you).

Plugins

Installing plugins for Refurb is very easy:

$ pip3 install refurb-plugin-example

Where refurb-plugin-example is the name of the plugin. Refurb will automatically load any installed plugins.

To make your own Refurb plugin, see the refurb-plugin-example repository for more info.

Writing Your Own Check

If you want to extend Refurb but don't want to make a full-fledged plugin, you can easily create a one-off check file with the refurb gen command.

Note that this command uses the fzf fuzzy-finder for getting user input, so you will need to install fzf before continuing.

Here is the basic overview for creating a new check using the refurb gen command:

1. First select the node type you want to accept
2. Then type in where you want to save the auto generated file
3. Add your code to the new file

To get an idea of what you need to add to your check, use the --debug flag to see the AST representation for a given file (ie, refurb --debug file.py). Take a look at the files in the refurb/checks/ folder for some examples.

Then, to load your new check, use refurb file.py --load your.path.here

Note that when using --load, you need to use dots in your argument, just like importing a normal python module. If your.path.here is a directory, all checks in that directory will be loaded. If it is a file, only that file will be loaded.

Developing

To setup locally, run:

$ git clone https://github.com/dosisod/refurb
$ cd refurb
$ make install
$ make install-local

Tests can be ran all at once using make, or you can run each tool on its own using make black, make flake8, and so on.

Unit tests can be ran with pytest or make test.

Since the end-to-end (e2e) tests are slow, they are not ran when running make. You will need to run make test-e2e to run them.

Why Does This Exist?

I love doing code reviews: I like taking something and making it better, faster, more elegant, and so on. Lots of static analysis tools already exist, but none of them seem to be focused on making code more elegant, more readable, or more modern. That is where Refurb comes in.

Refurb is heavily inspired by clippy, the built-in linter for Rust.

What Refurb Is Not

Refurb is not a style/type checker. It is not meant as a first-line of defense for linting and finding bugs, it is meant for making good code even better.

Comparison To Other Tools

There are already lots of tools out there for linting and analyzing Python code, so you might be wondering why Refurb exists (skepticism is good!). As mentioned above, Refurb checks for code which can be made more elegant, something that no other linters (that I have found) specialize in. Here is a list of similar linters and analyzers, and how they differ from Refurb:

Black: is more focused on the formatting and styling of the code (line length, trailing comas, indentation, and so on). It does a really good job of making other projects using Black look more or less the same. It doesn't do more complex things such as type checking or code smell/anti-pattern detection.

flake8: flake8 is also a linter, is very extensible, and performs a lot of semantic analysis-related checks as well, such as "unused variable", "break outside of a loop", and so on. It also checks PEP8 conformance. Refurb won't try and replace flake8, because chances are you are already using flake8 anyways.

Pylint has a lot of checks which cover a lot of ground, but in general, are focused on bad or buggy code, things which you probably didn't mean to do. Refurb assumes that you know what you are doing, and will try to cleanup what is already there the best it can.

Mypy, Pyright, Pyre, and Pytype are all type checkers, and basically just enforce types, ensures arguments match, functions are called in a type safe manner, and so on. They do much more then that, but that is the general idea. Refurb actually is built on top of Mypy, and uses its AST parser so that it gets good type information.

pyupgrade: Pyupgrade has a lot of good checks for upgrading your older Python code to the newer syntax, which is really useful. Where Refurb differs is that Pyupgrade is more focused on upgrading your code to the newer version, whereas Refurb is more focused on cleaning up and simplifying what is already there.

In conclusion, Refurb doesn't want you to throw out your old tools, since they cover different areas of your code, and all serve a different purpose. Refurb is meant to be used in conjunction with the above tools.