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Rule-defined fix

Rule-defined fix is a Semgrep feature that lets you add suggested fixes to rules.

Semgrep's rule format supports a fix: key that supports the replacement of metavariables and regex matches with potential fixes. When rules that include a Rule-defined fix are triggered, Semgrep suggests these fixes in your pull request or merge request comments. You can view and easily resolve the findings as part of your code review workflow.

You can apply the Rule-defined fix directly to the file using the --autofix flag. To test the fix before applying it, use both the --autofix and --dryrun flags.

tip

Rule-defined fixes are deterministic and user-defined. Related AI-powered features include Semgrep Assistant's Suggested fix, which identifies code that needs to be fixed, and Semgrep Autofix, which can automatically generate a fix PR with a single click.

Example Rule-defined fix snippet​

Sample Rule-defined fix (view in Playground):

rules:
- id: use-sys-exit
  languages:
  - python
  message: |
    Use `sys.exit` over the python shell `exit` built-in. `exit` is a helper
    for the interactive shell and is not available on all Python implementations.
    https://stackoverflow.com/a/6501134
  pattern: exit($X)
  fix: sys.exit($X)
  severity: MEDIUM

Create Rule-defined fix rules​

See how to create a Rule-defined fix in the Transforming code with Semgrep's Rule-defined fix video:

Rule-defined fix with regular expression replacement​

A variant of the fix key is fix-regex, which applies regular expression replacements (similar to sed) to matches found by Semgrep.

fix-regex has two required fields:

  • regex specifies the regular expression to replace within the match found by Semgrep
  • replacement specifies what to replace the regular expression with.

fix-regex also takes an optional count field, which specifies how many occurrences of regex to replace with replacement, from left-to-right and top-to-bottom. By default, fix-regex will replace all occurrences of regex. If regex does not match anything, no replacements are made.

The replacement behavior is identical to the re.sub function in Python. See these Python docs for more information.

An example rule with fix-regex is shown below. regex uses a capture group to greedily capture everything up to the final parenthesis in the match found by Semgrep. replacement replaces this with everything in the capture group (\1), a comma, timeout=30, and a closing parenthesis. Effectively, this adds timeout=30 to the end of every match.

rules:
- id: python.requests.best-practice.use-timeout.use-timeout
  patterns:
  - pattern-not: requests.$W(..., timeout=$N, ...)
  - pattern-not: requests.$W(..., **$KWARGS)
  - pattern-either:
    - pattern: requests.request(...)
    - pattern: requests.get(...)
    - pattern: requests.post(...)
    - pattern: requests.put(...)
    - pattern: requests.delete(...)
    - pattern: requests.head(...)
    - pattern: requests.patch(...)
  fix-regex:
    regex: '(.*)\)'
    replacement: '\1, timeout=30)'
  message: |
    'requests' calls default to waiting until the connection is closed.
    This means a 'requests' call without a timeout will hang the program
    if a response is never received. Consider setting a timeout for all
    'requests'.
  languages: [python]
  severity: MEDIUM

Remove a code detected by a rule​

Improve your code quality by cleaning up stale code automatically. To remove code idetified by a Rule-defined fix, add the fix key with an empty string "".

For example:

 - id: python-typing
   pattern: from typing import $X
   fix: ""
   languages: [ python ]
   message: found one
   severity: ERROR

When you apply this Rule-defined fix, the detected code is removed.

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