The fetter command-line tool scans and validates Python packages across virtual environments or entire systems, ensuring packages conform to specified requirements or lock files. It identifies unapproved or vulnerable packages, supports continuous integration through 'pre-commit', and offers excellent performance thanks to a multi-threaded Rust implementation.

• 🔎 System Scanning: Finds Python packages across system environments.
• ⚖️ Package Validation: Checks installed packages against requirements.txt, pyproject.toml, or lock files sourced locally, via URLs, or via git repositories.
• 🛡️ Vulnerability Audit: Scans packages for security vulnerabilites in the Open Source Vulnerability database.
• ⚙️ CI Integration: Validate and audit with pre-commit hooks.
• 🚀 Fast: Multi-threaded Rust implementation.
• 🪢 Bound Requirements: Derive lock-file-like bound requirements from observed system packages.
• 🧹 Search and Purge: Find and remove packages across environments.
• 🧩 Flexible Output: Display results in terminal or export to delimited files.

While available as a pure Rust binary (crates), fetter is easily installed via a Python package (pypi):

$ pip install fetter
$ fetter --help

Alternatively, as fetter can operate on multiple virtual environments, installation via pipx might be desirable:

$ pipx install fetter
$ fetter --version

For complete command-line documentation, see CLI Documentation.

By default, fetter will search for all packages in site-packages directories discoverable from all Python executables found in the system or user virtual environments. Depending on your system, this command might take several seconds.

$ fetter scan

To limit scanning to site-packages directories associated with a specific Python executable, the --exe (or -e) argument can be supplied.

$ fetter -e python3 scan
Package                   Site
certifi-2024.8.30         ~/.env-wp/lib/python3.12/site-packages
charset_normalizer-3.4.0  ~/.env-wp/lib/python3.12/site-packages
idna-3.10                 ~/.env-wp/lib/python3.12/site-packages
jinja2-3.1.3              ~/.env-wp/lib/python3.12/site-packages
markupsafe-2.1.5          ~/.env-wp/lib/python3.12/site-packages
pip-21.1.1                ~/.env-wp/lib/python3.12/site-packages
requests-2.32.3           ~/.env-wp/lib/python3.12/site-packages
setuptools-56.0.0         ~/.env-wp/lib/python3.12/site-packages
urllib3-2.2.3             ~/.env-wp/lib/python3.12/site-packages
zipp-3.18.1               ~/.env-wp/lib/python3.12/site-packages

This evnironment was built from this "requirements.txt":

jinja2==3.1.3
zipp==3.18.1
requests==2.32.3

To validate that the installed packages match the packages specified in "requirements.txt", we can use the fetter validate command, again targeting our active Python with -e python3, and providing "requirements.txt" to the --bound argument.

$ fetter -e python3 validate --bound requirements.txt
Package                   Dependency  Explain     Sites
certifi-2024.8.30                     Unrequired  ~/.env-wp/lib/python3.12/site-packages
charset_normalizer-3.4.0              Unrequired  ~/.env-wp/lib/python3.12/site-packages
idna-3.10                             Unrequired  ~/.env-wp/lib/python3.12/site-packages
markupsafe-2.1.5                      Unrequired  ~/.env-wp/lib/python3.12/site-packages
pip-21.1.1                            Unrequired  ~/.env-wp/lib/python3.12/site-packages
setuptools-56.0.0                     Unrequired  ~/.env-wp/lib/python3.12/site-packages
urllib3-2.2.3                         Unrequired  ~/.env-wp/lib/python3.12/site-packages

The --superset command can be provided to accept packages that are not defined in the bound requirements.

$ fetter -e python3 validate --bound requirements.txt --superset

If we update zipp to version 3.20.2 and re-run validation, fetter will report these as "Misdefined" records

$ fetter -e python3 validate --bound requirements.txt --superset
Package      Dependency    Explain     Sites
zipp-3.20.2  zipp==3.18.1  Misdefined  ~/.env-wp/lib/python3.12/site-packages

If we remove the zipp package entirely, fetter identifies this as a "Missing" record:

$ fetter -e python3 validate --bound requirements.txt --superset
Package  Dependency    Explain  Sites
         zipp==3.18.1  Missing

If we want to permit the absence of specified packages, the --subset flag can be used:

$ fetter -e python3 validate --bound requirements.txt --superset --subset

Using the fetter audit command, details are provided for every vulnerability associated with installed packages.

$ fetter -e python3 audit
Package            Vulnerabilities      Attribute  Value
jinja2-3.1.3       GHSA-h75v-3vvj-5mfj  URL        https://osv.dev/vulnerability/GHSA-h75v-3vvj-5mfj
                                        Summary    Jinja vulnerable to HTML attribute injection when passing ...
                                        Reference  https://nvd.nist.gov/vuln/detail/CVE-2024-34064
                                        Severity   CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N
pip-21.1.1         GHSA-mq26-g339-26xf  URL        https://osv.dev/vulnerability/GHSA-mq26-g339-26xf
                                        Summary    Command Injection in pip when used with Mercurial
                                        Reference  https://nvd.nist.gov/vuln/detail/CVE-2023-5752
                                        Severity   CVSS:4.0/AV:L/AC:L/AT:N/PR:L/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA
                   PYSEC-2023-228       URL        https://osv.dev/vulnerability/PYSEC-2023-228
                                        Reference  https://mail.python.org/archives/list/security-announce@py...
                                        Severity   CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:N
setuptools-56.0.0  GHSA-cx63-2mw6-8hw5  URL        https://osv.dev/vulnerability/GHSA-cx63-2mw6-8hw5
                                        Summary    setuptools vulnerable to Command Injection via package URL
                                        Reference  https://nvd.nist.gov/vuln/detail/CVE-2024-6345
                                        Severity   CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:A/VC:H/VI:H/VA:H/SC:N/SI:N/SA
                   GHSA-r9hx-vwmv-q579  URL        https://osv.dev/vulnerability/GHSA-r9hx-vwmv-q579
                                        Summary    pypa/setuptools vulnerable to Regular Expression Denial of...
                                        Reference  https://nvd.nist.gov/vuln/detail/CVE-2022-40897
                                        Severity   CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
                   PYSEC-2022-43012     URL        https://osv.dev/vulnerability/PYSEC-2022-43012
                                        Reference  https://github.com/pypa/setuptools/blob/fe8a98e696241487ba...
zipp-3.18.1        GHSA-jfmj-5v4g-7637  URL        https://osv.dev/vulnerability/GHSA-jfmj-5v4g-7637
                                        Summary    zipp Denial of Service vulnerability
                                        Reference  https://nvd.nist.gov/vuln/detail/CVE-2024-5569
                                        Severity   CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA

For additional discussion and examples of fetter commands and functionality, see System-Wide Python Package Control.

Two fetter commands can be run via pre-commit hooks for continuous integration of Python package controls.

The fetter validate command permits validating that the actually installed Python packages in the current environment are what are defined to be installed, as specified by a requirements.txt file, a pyproject.toml file, or a lock file such as one produced by uv.

The fetter validate command takes a required argument, --bound, to specify that path or URL to the file to be used to define the bound requirements. The optional --superset argument permits packages not defined in the bound requirements to be present. The optional --subset argument permits not all packages in the bound requirements to be present.

To run fetter validate with pre-commit, add the following to your .pre-commit-config.yaml.

repos:
- repo: https://github.com/fetter-io/fetter-rs
  rev: v1.1.0
  hooks:
    - id: fetter-validate
      args: [--bound, {FILE}, --superset, --subset]

The fetter audit command will check for cybersecurity vulnerabilities issued for all installed Python packages in the current environment. Vulnerabilities are searched for in the Open Source Vulnerability (OSV) database.

To run fetter audit with pre-commit, add the following to your .pre-commit-config.yaml. Note that, as searching vulnerabilities can take time, this hook is likely better deployed as a pre-push rather than a pre-commit hook.

repos:
- repo: https://github.com/fetter-io/fetter-rs
  rev: v1.1.0
  hooks:
    - id: fetter-audit

• --exe, -e <FILES>: Provide zero or more executable paths to derive site package locations. If omitted, all discoverable executables will be used.
• --quiet, -q: Disable logging and terminal animation.
• --user_site: Force inclusion of the user site-packages, even if it is not activated. Defaults to only including if the interpreter is configured to use it.

• Description: Scan the environment to report on installed packages.
• Subcommands
  • display: Show scan results in the terminal.
  • write: Save scan results to a file.
    • --output, -o <FILE>: Specify the output file.
    • --delimiter, -d <char>: Set the delimiter for the file (default: ,).

• Description: Search the environment to report on installed packages based on a pattern.
• Options
  • --pattern, -p <STRING>: Specify a glob-like pattern to match packages.
  • --case: Enable case-sensitive pattern matching.
• Subcommands
  • display: Show search results in the terminal.
  • write: Save search results to a file.
    • --output, -o <FILE>: Specify the output file.
    • --delimiter, -d <char>: Set the delimiter for the file (default: ,).

• Description: Count discovered executables, sites, and packages.
• Subcommands
  • display: Show count results in the terminal.
  • write: Save count results to a file.
    • --output, -o <FILE>: Specify the output file.
    • --delimiter, -d <char>: Set the delimiter for the file (default: ,).

• Description: Derive new requirements from discovered packages.
• Options
  • --anchor, -a <BOUND>: Set the nature of the bound in the derived requirements. (Use a value from CliAnchor)
• Subcommands
  • display: Show derived requirements in the terminal.
  • write: Save derived requirements to a file.
    • --output, -o <FILE>: Specify the output file.

• Description: Validate if packages conform to a specified validation target.
• Options
  • --bound, -b <FILE>: Path or URL to the file containing bound requirements.
  • --bound-options <OPTIONS>: Names of additional optional dependency groups.
  • --subset: Allow the observed packages to be a subset of the bound requirements.
  • --superset: Allow the observed packages to be a superset of the bound requirements.
• Subcommands
  • display: Show validation results in the terminal.
  • json: Print validation results in JSON format.
  • write: Save validation results to a file.
    • --output, -o <FILE>: Specify the output file.
    • --delimiter, -d <char>: Set the delimiter for the file (default: ,).
  • exit: Return an exit code (0 for success, customizable for errors).
    • --code, -c <INT>: Specify the error code (default: 3).

• Description: Search for security vulnerabilities in packages via the OSV DB.
• Options
  • --pattern, -p <STRING>: Specify a glob-like pattern to select packages (default: *).
  • --case: Enable case-sensitive pattern matching.
• Subcommands
  • display: Show audit results in the terminal.
  • write: Save audit results to a file.
    • --output, -o <FILE>: Specify the output file.
    • --delimiter, -d <char>: Set the delimiter for the file (default: ,).

• Description: Count all installed package artifacts.

• Options

  • --pattern, -p <STRING>: Specify a glob-like pattern to select packages (default: *).
  • --case: Enable case-sensitive pattern matching.

• Subcommands

  • display: Show artifact counts in the terminal.
  • write: Save artifact counts to a file.
    • --output, -o <FILE>: Specify the output file.
    • --delimiter, -d <char>: Set the delimiter for the file (default: ,).

• Description: List the file names of all installed package artifacts.
• Options
  • --pattern, -p <STRING>: Specify a glob-like pattern to select packages (default: *).
  • --case: Enable case-sensitive pattern matching.
• Subcommands
  • display: Show artifact file names in the terminal.
  • write: Save artifact file names to a file.
    • --output, -o <FILE>: Specify the output file.
    • --delimiter, -d <char>: Set the delimiter for the file (default: ,).

• Description: Purge packages that match a specific pattern.
• Options
  • --pattern, -p <STRING>: Specify a glob-like pattern to select packages (default: *).
  • --case: Enable case-sensitive pattern matching.

• Description: Purge packages that are invalid based on dependency specification.
• Options
  • --bound, -b <FILE>: Path or URL to the file containing bound requirements.
  • --bound-options <OPTIONS>: Names of additional optional dependency groups.
  • --subset: Allow the observed packages to be a subset of the bound requirements.
  • --superset: Allow the observed packages to be a superset of the bound requirements.

Implemented bound-options to permit selecting optional dependencies in pyproject.toml files.

Implemented home-path display abbreviation with ~.

Handle combining multiple DepSpec in producing DepManifest.

Added --pattern and --case options to audit.

Added support for creating DepManifest from pyproject.toml.

Permit DepManifest to be retrieved from a URL.

All subcommands now have their output sub-subcommands set to display by default.

The validate and audit subcommands now return a non-zero exit code when items are found.

The CLI now exits for unsupported platforms.

Extended validate and audit commands to return a non-zero error code if display prints records.

Implemented variable-width and colored terminal displays.

Implemented terminal spinner for long-running commands.

Added purge-invalid and purge-pattern commands.

Split unpack command into unpack-count and unpack-files.

Added support to specify --bound with a git repository.

Added --user-site flag to force inclusion of user site packages; otherwise, user site packages are only included if ENABLE_USER_SITE is set.

Reimplemented display and delimited table outputs to use a generic trait implementation.

Support --requirement in requirements files.

Package and DepSpec comparisons now remove user.

Package and DepSpec comparisons now accept matching either on requested_revision or commit_id.

URLs are now shown in DepSpec displays.

Delimited file output no longer pads spaces.

Validate display now shows paths properly.

Updated validate json output to terminate line and flush buffer.

Package and dependency keys are case insensitive.

Improved URL validation between dependency and package by removing user components.

Improved validation JSON output to provided labelled objects.

Improved valiation output to show sorted missing packages.

Renamed validation explain values.

Implemented support for nested requirements.txt.

Implemented search command with basic wildcard matching.

Implemented Arc-wrapped PathBuf for sharable site paths.

Added explanation column to validation results.

Added support for both --subset and --superset validations.

Implemented ValidationDigest for simplified JSON serialization.

Added JSON CLI output option for validation results.