Virtual environments — venv, pip, requirements.txt

1.3.12 · Coding › Python Intermediate

Virtual environments KYUN exist karte hain?

Teen cheezein jo venv isolate karta hai:

Packages — har env ka apna site-packages hota hai.
Python interpreter link — har env ek specific Python version ki taraf point karta hai.
pip/scripts — installed command-line tools env ke andar rehte hain.

VENV exactly kya hai? (First principles se derivation)

Jab Python start hota hai, wo imports search karne ke liye ek folder list banata hai, jise sys.path kehte hain. Key insight yeh hai:

Figure — Virtual environments — venv, pip, requirements.txt

KAISE use karein — poora workflow

Formula 6 core commands

# 1. CREATE an env named .venv in the current folder
python -m venv .venv
 
# 2. ACTIVATE it
source .venv/bin/activate        # Linux / macOS
.venv\Scripts\activate           # Windows (PowerShell/cmd)
 
# 3. INSTALL packages (go into .venv only)
pip install requests flask
 
# 4. FREEZE the exact versions to a file
pip freeze > requirements.txt
 
# 5. REBUILD elsewhere from that file
pip install -r requirements.txt
 
# 6. LEAVE the env
deactivate

Worked Example 1 — Ek fresh project banao

mkdir weather && cd weather
python -m venv .venv
source .venv/bin/activate
pip install requests
pip freeze > requirements.txt

Step	Yeh step kyun?
`mkdir weather`	Ek folder = ek project = ek env. Cheezein clean rehti hain.
`python -m venv .venv`	Isolated interpreter create karta hai. `.venv` conventional hidden naam hai.
`source .venv/bin/activate`	Ab `which python` → `weather/.venv/bin/python`, isliye installs yahan land karti hain.
`pip install requests`	`.venv/lib/.../site-packages` mein jaata hai, system Python mein nahi.
`pip freeze > requirements.txt`	`requests==2.32.3` (+ uski dependencies) record karta hai taaki teammate reproduce kar sake.

Worked Example 2 — Teammate ka project reproduce karo

git clone https://example/cool-app && cd cool-app
python -m venv .venv
source .venv/bin/activate
pip install -r requirements.txt

Step	Yeh step kyun?
`git clone`	Tumhe code aur `requirements.txt` milta hai, lekin env nahi (wo gitignored hai).
`python -m venv .venv`	Env local & disposable hai — ise rebuild karo, ship mat karo.
`pip install -r requirements.txt`	Author ne jo exact package set use kiya tha usse re-create karta hai. Same versions = same behaviour.

Worked Example 3 — `freeze` vs hand-written requirements

# pip freeze output (fully pinned, machine-generated)
flask==3.0.3
requests==2.32.3
certifi==2024.7.4      # ← transitive dependency, auto-included
 
# hand-written (loose, human-curated)
flask>=3.0
requests

Style	Kyun use karein?
`==` pins (from `freeze`)	Reproducible builds — production, exact state share karna.
`>=` / unpinned	Libraries, jahan tum chahte ho ki users compatible newer versions lein.

Common mistakes (Steel-manned)

Recall Feynman: ek 12-saal ke bachche ko explain karo

Socho tumhare computer ka Python ek shared toy box jaisa hai poore ghar ke liye. Agar tumhara bada bhai usme race-car parts bhardega aur tumhe doll parts chahiye, toh tum dono ek hi box ke liye ladte ho. Ek virtual environment aise hai jaise har bachche ko apna chhota box de do sirf unke toys ke liye. python -m venv ek naya khaali box banata hai. pip install toys sirf tumhare box mein rakhta hai. requirements.txt ek shopping list hai taaki agar tumhara box kho jaaye, tum exactly wahi toys phir se kharid sako. Aur activate ka bas matlab hai "abhi mera box kholo aur use karo".

Active-recall flashcards

Virtual environments kaunsi problem solve karte hain?

Dependency conflicts ("dependency hell") — har project ko apne package versions doosron se aur system Python se isolated rakhne deta hai.

.venv folder mein venv create karne ka command kya hai?

python -m venv .venv

venv ki jagah python -m venv kyun use karein?

-m us specific Python ke saath venv module run karta hai, fix karta hai ki env kaun sa interpreter version use karegi.

Venv activate karne se under the hood kya hota hai?

Shell PATH edit hoti hai taaki python aur pip env ki copies ki taraf point karein, installs ko env ke site-packages mein redirect karte hue.

Env ke exact packages ko file mein kaise save karein?

pip freeze > requirements.txt

Us file se env ko kaise rebuild karein?

pip install -r requirements.txt

Kya .venv/ ko git mein commit karna chahiye?

Nahi — ise gitignore karo; requirements.txt commit karo (env reproducible, OS-specific, aur bada hota hai).

Kya activation terminal windows ke across persist karta hai?

Nahi — yeh sirf current shell session ko affect karta hai; har naye terminal mein re-activate karo.

Requirements mein flask==3.0.3 aur flask>=3.0 mein kya difference hai?

== ek exact version pin karta hai (reproducible builds); >= compatible newer versions allow karta hai (libraries).

Active environment se kaise bahar niklen?

deactivate run karo.

Jab env active hota hai toh prompt mein kya dikhta hai?

Env ka naam parentheses mein, jaise (.venv).

site-packages kya hai?

Wo directory jahan installed third-party packages rehte hain; har venv ka apna hota hai.

Connections

pip and PyPI
Python import system and sys.path
Dependency management — poetry, pipenv
Reproducible builds and Docker
.gitignore best practices
Semantic versioning