Explain requirements.txt code

What this requirements.txt sample shows

pip install -r

pip install -r requirements.txt installs every line in one go. In a fresh virtual environment it printed Successfully installed for 10 packages: the 3 named here plus 7 they need, such as urllib3 and certifi for requests. Running it again changed nothing and printed Requirement already satisfied 10 times.

==, >= and <

==2.34.2 allows that one version only. >=1.0,<2 allows 1.0 and up to, but not including, 2.0: pip's own version checker accepted 1.0 and 1.2.4 and turned down 0.9 and 2.0, and pip installed python-dotenv 1.2.4.

~= compatible release

~=15.0 means 15.0 or newer, but still 15: the same checker accepted 15.0 and 15.3 and turned down 14.9 and 16.0. pip installed rich 15.0.0.

markers after ;

Text after ; is a condition about the machine. On this machine, which isn't Windows, pip printed Ignoring tzdata: markers 'sys_platform == "win32"' don't match your environment and skipped it. Python's zoneinfo docs recommend tzdata because Windows systems don't have an IANA time zone database.

Good to know

• Run with Python 3.14 and pip 26.2 in a fresh virtual environment, with the versions that were current on PyPI at the time.
• Two lines that can't both be true stop the install: requests==2.34.2 with requests>=3 gave ERROR: Cannot install requests==2.34.2 and requests>=3 because these package versions have conflicting dependencies.
• A version that doesn't exist fails with Could not find a version that satisfies the requirement requests==9.9.9, followed by the versions that do exist.
• pip uninstall requests removed only requests: urllib3, certifi and idna, which came in with it, stayed installed.

Watch out: pip freeze > requirements.txt writes down everything installed, not what you asked for. After installing these 4 lines, pip freeze listed 10 packages, each pinned with ==, including Pygments==2.21.0 and mdurl==0.1.2, which came in with rich and not from any line here. Keep the short file you wrote, and use freeze only when you want every version locked.