User Guide

The µsort command line interface is the primary method for sorting imports in your Python modules. Installing µsort can be done via pip:

$ pip install usort

To format one or more files or directories in-place:

$ usort format <path> [<path> ...]

To generate a diff of changes without modifying files:

$ usort diff <path> [<path> ...]

µsort can also be used to validate formatting as part of CI:

$ usort check <path> [<path> ...]

Sorting

µsort follows a few simple steps when sorting imports in a module:

1. Look for all import statements in the module

2. Group these statements into “blocks” of sortable imports (See Import Blocks for details)

3. Reorder import statements within each block

4. Normalize whitespace between imports as needed

When ordering imports within a block, µsort categorizes the imports by source into four major categories for imports, prioritized following common community standards:

• __future__ imports:

• Standard library modules (from CPython):

• Third-party modules (external imports)

• First-party modules (internal, local, or relative imports)

Within each category, imports are sorted first by “style” of import statement:

• “basic” imports (import foo)

• “from” imports (from foo import bar)

And lastly, imports of the same style are sorted lexicographically by source module name, and then by name of element being imported.

Altogether, this will result each block of imports sorted roughly according to this example, for a module in the namespace something:

# future imports
from __future__ import annotations

# standard library
import re
import sys
from datetime import date, datetime, timedelta
from pathlib import Path

# third-party
import requests
from attr import dataclasses
from honesty.api import download_many

# first-party
from something import other_function, some_function
from . import some_module
from .other_module import some_name, that_thing

Configuration

µsort shouldn’t require configuration for most projects, but offers some basic options to customize sorting and categorization behaviors.

pyproject.toml

The preferred method of configuring µsort is in your project’s pyproject.toml, in the tool.usort table. When sorting each file, µsort will look for the “nearest” pyproject.toml to the file being sorted, looking upwards until the project root is found, or until the root of the filesystem is reached.

[tool.usort]

The following options are valid for the main tool.usort table:

categories: List[str] = ["future", "standard_library", "third_party", "first_party"]

If given, this list of categories overrides the default list of categories that µsort provides. New categories may be added, but any of the default categories not listed here will be removed.

default_category: str = "third_party"

The default category to classify any modules that aren’t already known by µsort as part of the standard library or otherwise listed in the tool.usort.known table.

side_effect_modules: List[str]

An optional list of known modules that have dangerous import-time side effects. Any module in this list will create implicit block separators from any import statement matching one of these modules.

See Side Effect Imports.

[tool.usort.known]

The tool.usort.known table allows for providing a custom list of known modules for each category defined by categories above. These modules should be a list of module names assigned to a property named matching the category they should be assigned to. If a module is listed under multiple catergories, the last category it appears in will take precedence.

As an example, this creates a fifth category “numpy”, and adds both numpy and pandas to the known modules list for the “numpy” category, as well as adding the example module to the “first_party” category:

[tool.usort]
categories = ["future", "standard_library", numpy", "third_party", "first_party"]
default_category = "third_party"

[tool.usort.known]
numpy = ["numpy", "pandas"]
first_party = ["example"]

Import Blocks

µsort uses a set of simple heuristics to detect “blocks” of imports, and will only rearrange imports within these distinct blocks.

Comment Directives

Comments with special directives create explicit blocks, separated by the line containing the directives, which will remain unchanged:

import math

import important_thing  # usort: skip

import difflib

Both # usort:skip and # isort:skip (with any amount of whitespace), will trigger this behavior, so existing comments intended for isort will still work with µsort.

Statements

Any non-import statement positioned between imports will create an implicit block separator. This allows µsort to automatically preserve use of modules that must happen before other imports, such as filtering warnings or debug logging:

import warnings
warnings.filterwarnings(...)  # <-- implicit block separator

import noisy_module

print("in between imports")  # <-- implicit block separator

import other_module

Shadowed Imports

Any import that shadows a previous import will create an implicit block separator:

import foo as os
import os  # <-- implicit block separator

Star Imports

Star imports, which can potentially shadow or be shadowed by any other import, will also create implicit block separators:

import foo

from bar import *  # <-- implicit block separator

import dog

Side Effect Imports

Writing modules with import-time side effects is a bad practice; any side effects should ideally wait for a function in that module to be called, like with warnings.filterwarnings(). In these cases, µsort will correctly find and create a block separator, preventing accidental changes in execution order when sorting.

However, it’s common for testing libraries and entry points to have well-known side effects when imported, and this can cause trouble with import sorting. Rather than adding # usort:skip comments to every occurence, these modules can be added to the side_effect_modules configuration option:

[tool.usort]
side_effect_modules = ["sir_kibble"]

µsort will then treat any import of these modules as implicit block separators:

import foo

from sir_kibble import leash  # <-- implicit block separator

import dog

This may result in less-obvious sorting results for users unaware of the context, so it is recommended to use this sparingly. The list-imports command may be useful for understanding how this affects your source files.

Troubleshooting

If µsort behavior is unexpected, or you would like to see how µsort detects blocks in your code, the list-imports command may help.

Given the file test.py:

import warnings
warnings.filterwarnings(...)

import foo
from bar import bar  # usort:skip

import sys

Running list-imports will generate the following output:

$ usort list-imports test.py
test.py 3 blocks:
body[0:1]
Formatted:
[[[
import warnings
]]]
body[2:3]
Formatted:
[[[

import foo
]]]
body[4:5]
Formatted:
[[[

import sys
]]]

Note that imports that are also block separators (like star imports or imports with skip directives) will not be listed in the output, because they are not within the sortable blocks that µsort operates on.

If more details are desired, the --debug flag will also provide categories and sorting information for each import:

$ usort list-imports --debug test.py
test.py 3 blocks:
body[0:1]
    0 SortableImport(sort_key=SortKey(category_index=1, is_from_import=False, ndots=0), first_module='warnings', first_dotted_import='warnings', imported_names={'warnings'}) (Category.STANDARD_LIBRARY)
body[2:3]
    0 SortableImport(sort_key=SortKey(category_index=2, is_from_import=False, ndots=0), first_module='foo', first_dotted_import='foo', imported_names={'foo'}) (Category.THIRD_PARTY)
body[4:5]
    0 SortableImport(sort_key=SortKey(category_index=1, is_from_import=False, ndots=0), first_module='sys', first_dotted_import='sys', imported_names={'sys'}) (Category.STANDARD_LIBRARY)