Python Cheatsheet

Jul 17, 2024 • Cheatsheet/python • 5 min read

Python cheatsheet Create virtual environment $ python3 -m venv venv

(to delete old virtual envionrment) $ python3 -m venv venv --clear

To create multiple virtual environment $ python3 -m venv venv /Users/name/virtualenvs/venv-copy

Activate virtual environment Change to directory containing venv $ source venv/bin/activate

Deactivate virtual environment $ deactivate

pip install -r requirements.txt

pip freeze > requirements.txt

To ensure the required modules are loaded when running Python scripts from bash, you can use a shebang line at the top of your script and activate the virtual environment within the script. Here is a step-by-step guide:

Create and activate a virtual environment:

python -m venv myenv
source myenv/bin/activate

Install the required modules:
```
pip install -r requirements.txt
```
Add a shebang line to your Python script:
```
#!/usr/bin/env python
```

Modify the script to activate the virtual environment automatically:

#!/usr/bin/env python

import os
import sys

# Path to the virtual environment
venv_path = os.path.join(os.path.dirname(__file__), 'myenv', 'bin', 'activate_this.py')

with open(venv_path) as f:
    exec(f.read(), {'__file__': venv_path})

# Rest of your script

Make your script executable:
```
chmod +x your_script.py
```
Run the script from bash:
```
./your_script.py
```

By following these steps, your script will ensure the virtual environment is activated and the required modules are loaded before executing the rest of the script.

If you don't want to manually activate the virtual environment each time, you can automate it by wrapping your script execution within a bash script that ensures the virtual environment is activated. Here’s how to do it:

Create a bash script to run your Python script:

#!/bin/bash

# Path to your virtual environment
VENV_PATH="path/to/your/venv"

# Activate the virtual environment
source "$VENV_PATH/bin/activate"

# Run the Python script
python path/to/your_script.py

# Deactivate the virtual environment
deactivate

Make your bash script executable:
```
chmod +x run_my_script.sh
```
Run your bash script:
```
./run_my_script.sh
```

This way, the bash script ensures the virtual environment is activated before running your Python script and deactivated afterward. You only need to run the bash script, and it will handle the environment setup for you.

Yes, you can set environment variables within a Python script similarly to how you use export in bash. You can do this using the os module in Python. Here’s how you can do it:

Setting Environment Variables in a Python Script:

import os

# Set environment variables
os.environ['MY_VARIABLE'] = 'my_value'
os.environ['ANOTHER_VARIABLE'] = 'another_value'

# Verify by printing
print(os.environ['MY_VARIABLE'])
print(os.environ['ANOTHER_VARIABLE'])

Running External Commands with the Modified Environment:

If you need to run external commands or scripts that require these environment variables, you can use the subprocess module:

import os
import subprocess

# Set environment variables
os.environ['MY_VARIABLE'] = 'my_value'
os.environ['ANOTHER_VARIABLE'] = 'another_value'

# Run an external command
result = subprocess.run(['echo', '$MY_VARIABLE'], capture_output=True, text=True, shell=True)
print(result.stdout)

Using a Context Manager for Temporary Environment Variables:

If you only need the environment variables to be set temporarily for a specific block of code, you can use a context manager:

import os
from contextlib import contextmanager

@contextmanager
def set_env(**env_vars):
    original_env = os.environ.copy()
    os.environ.update(env_vars)
    try:
        yield
    finally:
        os.environ.clear()
        os.environ.update(original_env)

with set_env(MY_VARIABLE='my_value', ANOTHER_VARIABLE='another_value'):
    # Your code here
    print(os.environ['MY_VARIABLE'])
    print(os.environ['ANOTHER_VARIABLE'])

# Outside the context manager, the environment variables are reset
print(os.environ.get('MY_VARIABLE'))  # This will print None

These methods allow you to set and use environment variables within your Python script, similar to exporting them in bash.

Yes, you can preload a Python script similarly to how you would source a bash function in a shell script. This can be done by creating a Python module or script that sets up the environment or performs certain actions, and then importing or running it in your main script.

Here are a few methods to preload a Python script:

Method 1: Using Import Statements

Create a Python module (preload.py):

# preload.py

import os

# Set environment variables
os.environ['MY_VARIABLE'] = 'my_value'
os.environ['ANOTHER_VARIABLE'] = 'another_value'

def setup():
    print("Environment variables set!")

Import the module in your main script (main_script.py):

# main_script.py

import preload

# Run the setup function to preload settings
preload.setup()

# Your main script logic here
print(os.environ['MY_VARIABLE'])
print(os.environ['ANOTHER_VARIABLE'])

Method 2: Executing a Preload Script

Create a preload script (preload.py):

# preload.py

import os

# Set environment variables
os.environ['MY_VARIABLE'] = 'my_value'
os.environ['ANOTHER_VARIABLE'] = 'another_value'

print("Environment variables set!")

Execute the preload script in your main script (main_script.py):

# main_script.py

import os
import subprocess

# Execute the preload script
subprocess.run(['python', 'preload.py'])

# Your main script logic here
print(os.environ['MY_VARIABLE'])
print(os.environ['ANOTHER_VARIABLE'])

Method 3: Using an Initialization Function

Create a Python module with an initialization function (preload.py):

# preload.py

import os

def initialize():
    # Set environment variables
    os.environ['MY_VARIABLE'] = 'my_value'
    os.environ['ANOTHER_VARIABLE'] = 'another_value'
    print("Environment variables set!")

Call the initialization function in your main script (main_script.py):

# main_script.py

from preload import initialize

# Run the initialization function to preload settings
initialize()

# Your main script logic here
print(os.environ['MY_VARIABLE'])
print(os.environ['ANOTHER_VARIABLE'])

These methods allow you to preload settings or environment variables in a Python script, similar to preloading a function in a bash script. The most common and clean approach is using an initialization function or importing a module, as it keeps your code modular and easy to maintain.