Autofix

Hands down the best way to enforce a code standard is to just automatically fix it. Semgrep's rule format supports a fix: key that supports metavariable replacement, much like message fields. This allows for value capture and rewriting.

The autofix can be applied directly to the file using the --autofix flag, or you can use both the --autofix and --dryrun flags to test the autofix.

Example autofix (see in Playground here):

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 be available on all Python implementations.
    https://stackoverflow.com/a/6501134
  pattern: exit($X)
  fix: sys.exit($X)
  severity: WARNING

Creating autofix rules

See how to create an autofix rule in Transforming code with Semgrep autofixes video:

Autofix with regular expression replacement

A variant on the fix key is fix-regex, which applies regular expression replacements (think 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: WARNING

Removing a code detected by a rule

Improve your code quality by cleaning up stale code automatically. Remove code that an autofix rule detected by adding 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 an autofix is applied, this rule removes the detected code.