How to install Python packages from a requirements file

Installing packages from a requirements file is the normal way to rebuild an application's tested dependency set after cloning a repository, recreating a virtual environment, or preparing a CI job. Reading the package list from one maintained file keeps development, automation, and deployment environments aligned with the versions the project expects.

The install is driven by python3 -m pip install -r requirements.txt, where -r tells pip to read requirement specifiers from a file instead of treating the next token as one package name. Current pip documentation still defines requirements files as line-based input for package specifiers, nested -r includes, constraint files, and selected install options, so one file can represent more than a flat list of packages.

Interpreter selection is the most common failure point. Many distro-managed Python installations are now marked as externally managed, and package-source settings can also come from pip configuration, environment variables, or entries inside the requirements file itself. The steps below use a POSIX shell with an active virtual environment and python3; on Windows, activate the intended environment first and use py when the launcher is the normal entry point.

Steps to install Python packages from a requirements file with pip:

Activate the project virtual environment and confirm that the selected interpreter is the one that should receive the packages.
```
$ source .venv/bin/activate
$ python3 -m pip --version
pip 26.0.1 from /srv/apps/acme-api/.venv/lib/python3.14/site-packages/pip (python 3.14)
```
The path after from should point at the expected virtual environment or interpreter prefix before the install starts.

If the path points at an operating-system-managed Python, move the install into a virtual environment instead of forcing packages into the system interpreter.

Use .\.venv\Scripts\activate and py -m pip --version on Windows when the py launcher is the normal entry point.

Related: How to create a virtual environment in Python
Related: How to activate a Python virtual environment
Review the target requirements file before installing from it.
```
$ sed -n '1,20p' requirements.txt
idna==3.11
packaging==25.0
```
```
requirements.txt
```
is conventional rather than required; keep the actual relative or absolute path after -r when the project uses another filename.
Requirements files can contain package specifiers, comments, nested -r includes, constraints with -c, and supported global options such as --index-url.
Install the packages declared in the requirements file.
```
$ python3 -m pip install -r requirements.txt
Collecting idna==3.11 (from -r requirements.txt (line 1))
Collecting packaging==25.0 (from -r requirements.txt (line 2))
Installing collected packages: packaging, idna

Successfully installed idna-3.11 packaging-25.0
```
pip resolves the full requirement set and installs any additional dependencies the declared packages need; the exact install order can vary with the resolved dependency graph.

If pip reports that no matching distribution could be found for a package that should exist, inspect the active package index, proxy, and authentication settings before changing the pinned version.

Related: How to configure a pip index URL
Related: How to configure a proxy for pip

Verify that one or more packages from the file landed in the expected environment.

$ python3 -m pip show idna packaging
Name: idna
Version: 3.11
Summary: Internationalized Domain Names in Applications (IDNA)
Location: /srv/apps/acme-api/.venv/lib/python3.14/site-packages
Requires:
Required-by:
---
Name: packaging
Version: 25.0
Summary: Core utilities for Python packages
Location: /srv/apps/acme-api/.venv/lib/python3.14/site-packages
Requires:
Required-by:

The Location field confirms whether the install landed in the project environment, a user-site directory, or another interpreter context.

Run pip check after the install to catch unsatisfied or conflicting dependencies in the active environment.
```
$ python3 -m pip check
No broken requirements found.
```
If pip check reports broken requirements, fix the package source or version pins before treating the environment as reusable.