pip

pip is the standard package installer for Python, managing dependencies from PyPI while adhering to modern packaging standards and isolated environment constraints.

When to Use

• Python Package Installation & Management: Installing, updating, and isolating Python dependencies from PyPI.
• Deterministic Dependency Pinning: Enforcing exact versions with pip-tools and constraints files (-c constraints.txt).
• Modern Packaging Standards (PEP 517/621): Installing projects built with pyproject.toml without legacy setup.py.
• Security & Integrity Verification: Installing packages with cryptographic SHA256 hashes (--require-hashes).

Quick Start

python -m venv .venv
source .venv/bin/activate

pip install requests
pip freeze > requirements.txt

Core Concepts

Deterministic Installation with Constraints Files

Separating direct top-level requirements from strictly pinned transitive constraints:

# requirements.txt (Direct dependencies)
fastapi
pydantic
uvicorn[standard]
sqlalchemy

# constraints.txt (Pinned transitive tree with hashes)
annotated-types==0.7.0 --hash=sha256:3bf9499874a...
fastapi==0.115.0 --hash=sha256:91823746198...
pydantic==2.9.2 --hash=sha256:d8912374691...

# Install direct requirements locked to verified constraint hashes
pip install --no-deps --require-hashes -r constraints.txt
pip install --no-cache-dir -r requirements.txt -c constraints.txt

Editable Development Installation with pyproject.toml

Installing local development libraries into virtual environments:

# Editable install adhering to modern PEP 660
pip install -e .[dev,test]

Auditing Installed Packages for Vulnerabilities

Checking dependencies against known security advisories:

# Check installed packages against PyPI Advisory Database using pip-audit
pip install pip-audit
pip-audit --desc

Common Patterns

Deterministic Dependency Pinning with pip-tools

Problem: Unpinned transitive dependencies in requirements.txt cause builds to break unexpectedly.

Solution: Compile pinned dependency locks from abstract requirements:

# 1. Define top-level dependencies in requirements.in:
echo "fastapi>=0.110.0
uvicorn[standard]
pydantic" > requirements.in

# 2. Compile fully pinned lockfile with exact hashes:
pip-compile --generate-hashes requirements.in -o requirements.txt

# 3. Synchronize environment strictly matching lockfile:
pip-sync requirements.txt

Best Practices

Do:

• Always install packages into an isolated virtual environment (python -m venv .venv), never into the global system Python.
• Use pip install --no-cache-dir in Dockerfiles to minimize container image sizes.
• Enforce --require-hashes in production deployments to prevent supply-chain package tampering.
• Use pip-audit in CI to detect vulnerable dependencies before merging pull requests.

Don't:

• Run sudo pip install; modifying the system Python environment can break OS packages.
• Maintain unpinned requirements.txt in production; use pip-compile (pip-tools) to generate locked files.
• Use legacy setup.py install; use pip install . adhering to modern pyproject.toml standards.

Troubleshooting

Error	Cause	Solution
error: externally-managed-environment	PEP 668 preventing direct pip install into system Python on Linux/macOS.	Create and activate a virtual environment: python3 -m venv .venv && source .venv/bin/activate.
Could not find a version that satisfies the requirement	Package name typo or incompatible Python version constraints.	Verify package name on PyPI and check supported Python versions.
Failed building wheel for ...	Missing C compiler, header files, or python development headers.	Install build-essential and python3-dev packages.

References

• pip Documentation