emend 0.2.1

A Python refactoring CLI tool with structured edits and pattern transforms

emend

A Python refactoring CLI built on LibCST. The name means "to make corrections to a text" — which is what it does, but with AST-aware precision instead of find-and-replace.

Two complementary systems: structured edits use selectors like file.py::func[params][0] for precise changes to symbol metadata, and pattern transforms use capture variables like print($X) → logger.info($X) for code-pattern search and replace.

Installation

Using uv with free-threaded Python (recommended for best performance)

uv tool install --python 3.14t emend

Python 3.13+ ships a free-threaded variant (3.13t, 3.14t) that removes the GIL. emend's Rust core (emend_core) is already GIL-free (built with #[pymodule(gil_used = false)]), so on free-threaded Python it can run parallel file scans with no lock contention — meaning search, lint, refs, and rename across large codebases are significantly faster.

The --python 3.14t flag tells uv to use the free-threaded interpreter. Use 3.13t if you prefer the stable free-threaded release:

uv tool install --python 3.13t emend   # free-threaded 3.13 (stable)
uv tool install --python 3.14t emend   # free-threaded 3.14 (latest)

Using uv (standard Python)

uv tool install emend

Using pip

pip install emend

Run emend --help to verify. Full documentation at lucaswiman.github.io/emend.

Usage

emend <command> [options]

Workflow

All mutating commands default to dry-run, showing a diff of proposed changes. Re-run with --apply to write them. You'll probably want to run a formatter (black/ruff/isort) afterward, since emend doesn't try to preserve exact formatting.

Selector Syntax

Three types of selectors:

Symbol Selectors

file.py::Class.method.nested   # Nested symbol path
file.py::func                  # Module-level symbol

Extended Selectors (with components)

file.py::func[params]           # Function parameters
file.py::func[params][ctx]      # Specific parameter (by name)
file.py::func[params][0]        # Specific parameter (by index)
file.py::func[returns]          # Return annotation
file.py::func[decorators]       # Decorator list
file.py::MyClass[bases]         # Base classes
file.py::func[body]             # Function body

Pseudo-class Selectors

file.py::func[params]:KEYWORD_ONLY       # Keyword-only parameter slot
file.py::func[params]:POSITIONAL_ONLY    # Positional-only parameter slot

Line Selectors

file.py:42                      # Single line
file.py:42-100                  # Line range

Wildcard Selectors

file.py::*[params]              # All function parameters
file.py::Test*[decorators]      # Test class parameters
file.py::*.*[returns]           # All method return types
file.py::Class.*[body]          # All method bodies in Class

Wildcards support glob patterns:

• * - Match any symbol at this level
• Test* - Match symbols starting with Test
• *.* - Match any method in any class
• Class.* - Match any method in Class

Selector Grammar (Lark)

start: selector

selector: file_path DOUBLE_COLON symbol_path? component*

file_path: PATH
symbol_path: symbol_segment ("." symbol_segment)*
symbol_segment: WILDCARD | IDENTIFIER
component: "[" COMPONENT_NAME "]" accessor? pseudo_class?
accessor: "[" (IDENTIFIER | INT) "]"
pseudo_class: PSEUDO_CLASS

COMPONENT_NAME: "params" | "returns" | "decorators" | "bases" | "body" | "imports"
DOUBLE_COLON: "::"
PATH: /[^:]+/
WILDCARD: "*" | /[a-zA-Z_*][a-zA-Z0-9_*]*/
IDENTIFIER: /[a-zA-Z_][a-zA-Z0-9_]*/
INT: /-?\d+/
PSEUDO_CLASS: /:KEYWORD_ONLY|:POSITIONAL_ONLY|:POSITIONAL_OR_KEYWORD/

Commands

Search & Read

search - Unified search with auto-detection

• Pattern mode: emend search 'print($X)' file.py
• Lookup mode: emend search file.py::func
• Summary mode: emend search file.py (list symbols)
• Filters: --kind, --name, --returns, --depth, --has-param, --output, --where, --imported-from, --scope-local
• Output formats: code, location, selector, summary, metadata, json, count, summary::flat, code::dedent
• Also available as: query, show, get, lookup, find for intuitive workflows

Edit & Transform

edit - Modify or remove existing symbol components

• Replace: emend edit file.py::func[returns] "int" --apply
• Insert: emend edit file.py::func[params] "new_param" --before ctx --apply
• Remove: emend edit file.py::func[params][old_param] --rm --apply
• Wildcards: emend edit 'file.py::*[decorators]' "@dataclass" --apply

add - Insert new items into list components (alternative to edit)

• emend add file.py::func[params] "timeout: int = 30" --apply
• emend add "file.py::func[params]:KEYWORD_ONLY" "debug: bool" --apply

replace - Replace pattern matches with pattern-based substitution

• emend replace 'print($X)' 'logger.info($X)' file.py --apply
• Scope: --where (syntax: 'def', 'class', 'MyClass.method', 'not ...')

Symbol Management

refs - Find all references to a symbol across the project

• emend refs models.py::User
• Filters: --writes-only, --reads-only, --calls-only
• Output: --json for JSON output (default shows file:line)

rename - Rename a symbol or module across the project

• Symbol: emend rename models.py::User --to Account --apply
• Module: emend rename models.py --to accounts.py --apply
• Filters: --docs, --no-hierarchy, --unsure

move - Move a symbol or module with automatic import updates

• Symbol: emend move utils.py::parse_date helpers/dates.py --apply
• Module: emend move utils.py helpers/utils.py --apply

copy-to - Copy a symbol to another file

• emend copy-to workflow.py::Builder._build.helper tasks.py --dedent --apply

Utilities

batch - Apply multiple refactoring operations from YAML/JSON file

• emend batch rules.json --apply

lint - Lint files using pattern rules from .emend/patterns.yaml

• emend lint src/
• emend lint src/ --fix to auto-fix issues
• emend lint src/ --rule no-print to run a single rule
• See Linting documentation for full details

deadcode - Find potentially dead (unreferenced) code

• emend deadcode src/
• emend deadcode . --kind function --json
• emend deadcode src/ --exclude-references-from tests/

graph - Generate a call graph for functions in a file

• emend graph src/module.py --format plain
• Formats: plain, json, dot

Examples

Search & Read Examples

# Search by pattern (pattern mode)
emend search 'print($X)' src/
emend search 'assertEqual($A, $B)' tests/ --output count

# Search by symbol (lookup mode)
emend search file.py::func
emend search src/ --kind function --name test_*
emend search file.py --output json

# Extract function parameters
emend search api.py::handler[params]
emend search 'api.py::*[params]'  # Wildcard: all function params in file

# Get return types
emend search 'src/**/*.py::*[returns]' --output metadata

# List symbols in a module
emend search file.py                          # Tree view
emend search file.py --output summary::flat   # Flat list
emend search file.py --depth 2                # Limit nesting depth

Edit Examples

# Update return type
emend edit api.py::handler[returns] "Response" --apply

# Add parameter with default value
emend edit api.py::handler[params] "timeout: int = 30" --apply

# Add keyword-only parameter
emend edit "api.py::handler[params]:KEYWORD_ONLY" "debug: bool" --apply

# Insert parameter before specific param
emend edit api.py::handler[params] "ctx: Context" --before user_id --apply

# Remove a specific parameter
emend edit api.py::handler[params][deprecated_arg] --rm --apply

# Edit multiple symbols at once (wildcards)
emend edit 'file.py::*[decorators]' "@dataclass" --apply

Pattern Transform Examples

# Simple find and replace (dry-run by default)
emend replace 'print($X)' 'logger.info($X)' file.py

# Replace within a specific scope
emend replace 'old_var' 'new_var' api.py --where process --apply

# Replace with pattern capture
emend replace 'get_field($N)' 'field$N' api.py --where process --apply

# String content interpolation: ${X.content} strips quotes from a captured string literal
emend replace 'Union["$X", $Y]' '$X | $Y' src/ --apply

# Find all pattern matches
emend search 'print($X)' src/ --output location

# Multi-rule batch operations
emend batch rules.json --apply

Symbol Management Examples

# Find all references to a symbol
emend refs models.py::User --json
emend refs models.py::User --writes-only    # Only write references
emend refs models.py::User --calls-only     # Only function calls

# Rename a symbol project-wide
emend rename models.py::User --to Account --apply

# Move a symbol to another file (updates imports)
emend move utils.py::parse_date helpers/dates.py --apply

# Copy a symbol to another file
emend copy-to workflow.py::Builder._build.helper tasks.py --dedent --apply

# List symbols using search
emend search workflow.py --depth 3

Pattern Syntax

Patterns support metavariables for capturing:

# Single expression
emend find 'print($MSG)' src/

# Multiple arguments with capture
emend find 'func($A, $B)' src/

# Variable arguments
emend find 'func($...ARGS)' src/

# Type constraints
emend find 'range($N:int)' src/

# Anonymous metavariables
emend find 'func($_, $ARG)' src/

# Structural constraints (via --where)
emend find 'print($X)' src/ --where 'async def'
emend find 'await $X' src/ --where 'not if __debug__'

# Supported pattern types:
#   Literals: $X, $MSG:str, $N:int, 3.14
#   Calls: func($X), obj.method($A, $B)
#   Operations: $A + $B, $A and $B, not $X, $X[$Y]
#   Collections: ($A, $B), [$X, $Y], {$K: $V}
#   Control: return $X, assert $A == $B, raise $EXC

Pattern Grammar (Lark)

start: pattern

pattern: (code_chunk | metavar)+

metavar: DOLLAR (ELLIPSIS)? METAVAR_NAME TYPE_CONSTRAINT?
       | DOLLAR UNDERSCORE

DOLLAR: "$"
ELLIPSIS: "..."
UNDERSCORE: "_"
METAVAR_NAME: /[A-Z][A-Z0-9_]*/
TYPE_CONSTRAINT: /:!?(?:expr|stmt|identifier|int|str|float|call|attr|any)/
code_chunk: /[^$:]+/ | ":"

The code_chunk rule excludes colons (/[^$:]+/) to prevent consuming colons that are part of type constraints (e.g., $MSG:str). A standalone colon is matched by the alternative | ":" for patterns containing colons outside of type constraints.

Diff Patch Format

- pattern_to_find
+ replacement_pattern

- another_pattern
+ another_replacement

Lines prefixed with - are matched; corresponding + lines are the replacement. Blank lines separate rules.

Linting

emend includes a pattern-based linter. Define rules in .emend/patterns.yaml and check your code for violations:

# .emend/patterns.yaml
macros:
  print_call: "print($...ARGS)"

rules:
  no-print:
    find: "{print_call}"
    not-inside: "def test_*"
    message: "Use logger instead of print"
    replace: "logger.info($...ARGS)"

  no-open-without-encoding:
    find: "open($PATH)"
    message: "Specify encoding when calling open()"
    replace: "open($PATH, encoding='utf-8')"

# Check for violations
emend lint src/

# Auto-fix violations that have a replace rule
emend lint src/ --fix

# Run only a specific rule
emend lint src/ --rule no-print

Suppress violations inline with # noqa comments:

print("keep this")  # noqa
print("keep this")  # noqa: emend:no-print
print("keep this")  # noqa: E501, emend:no-print  # mixed with other linters

Dead code detection

emend includes a deadcode section in .emend/patterns.yaml to detect unreferenced symbols as part of linting:

# .emend/patterns.yaml
deadcode: true

# Or with options:
deadcode:
  enabled: true
  kind: function                          # Only functions (or "class")
  exclude-references-from: ["tests/"]     # Ignore refs from tests
  include-private: false                  # Skip _private symbols
  strings-count-as-references: true       # String literals count as refs
  message: "Symbol appears to be unused"

# Run as part of lint
emend lint src/

# Or use the standalone command
emend deadcode src/
emend deadcode src/ --exclude-references-from tests/ --json

Suppress false positives inline:

def my_entry_point():  # noqa: emend:deadcode
    ...

pre-commit integration

emend can run as a pre-commit hook. Add to your .pre-commit-config.yaml:

repos:
  - repo: https://github.com/lucaswiman/emend
    rev: v0.2.0  # replace with desired version tag
    hooks:
      - id: emend-lint

This runs emend lint on staged Python files using your .emend/patterns.yaml config.

To auto-fix violations, add args: ["--fix"] to the hook configuration.

Development

Installing from Source

Clone the repository and install for development:

git clone https://github.com/lucaswiman/emend
cd emend

# Using make (creates a 3.14t venv, compiles the Rust extension, installs dev deps)
make venv

# Or manually (requires maturin and a Rust toolchain):
uv venv .venv --python 3.14t
uv pip install maturin
.venv/bin/maturin develop -E dev

Running Tests

# Run all tests
make test

# Run specific test file
make test TESTS=tests/test_emend/test_add_parameter.py

# Run specific test
make test TESTS="tests/test_emend/test_add_parameter.py::test_add_parameter_with_default"

Project Structure

emend/
├── src/emend/
│   ├── cli.py                # CLI entry point, argument parsing
│   ├── transform.py          # Transform primitives (get/set/add/remove/find/replace)
│   ├── pattern.py            # Pattern parsing and compilation
│   ├── query.py              # Symbol querying with filters
│   ├── ast_commands.py       # AST-based command implementations
│   ├── ast_utils.py          # AST traversal utilities
│   ├── component_selector.py # Extended selector parsing
│   ├── lint.py               # Pattern-based linter engine
│   └── grammars/
│       ├── selector.lark     # Extended selector grammar
│       └── pattern.lark      # Pattern grammar
├── rust/                     # emend_core Rust extension (bundled in wheel)
│   ├── src/lib.rs            # PyO3 bindings, GIL-free module definition
│   └── Cargo.toml
├── tests/test_emend/         # Test suite
├── Makefile
└── pyproject.toml            # maturin build (bundles Rust + Python in one wheel)

License

MPL 2.0