Should I put #! (shebang) in Python scripts, and what form should it take?

Shebang - What and why?

The shebang line (#!/usr/bin/env python3) is the interpreter directive for your Python script. It helps the system understand what interpreter to use to parse the script. In essence, it says: "Hey, I'm a Python 3 script!"

Omitting shebang - Is it Okay?

You might want to skip the shebang if your script will always be run explicitly invoking Python or if you're sure about the target system where the script will be running. However, if your script is for distribution or system operations, ensure to include a shebang.

Dealing with virtual environments

In the context of virtual environments, the shebang #!/usr/bin/env python is practical. It helps your script to run with the virtual environment's interpreter rather than your global Python interpreter. Think of it as the script’s courtesy to its present environment.

Windows and the Shebang

Windows platforms also respect shebangs, particularly with the Python Launcher for Windows. Include a shebang in scenarios where you're using cross-OS tools like WSL or Cygwin, or if your scripts are for Unix-users.

Legacy systems - Python 2.x considerations

For scripts aiming to offer support on legacy systems that still use Python 2.x, the shebang must be flexible. Specify the Python version in the shebang or within the script to ensure error-free execution.

Package designs and shebangs

Where script files are part of a Python package being installed via setuptools or similar package tools, a Python interpreter can be specified in the shebang. However, packaging tools generally manage the shebang when installing the package.

Project Specifics - The Django vs Tornado Scenario

Looking at open-source projects like Tornado, which includes a shebang, against Django, where usage isn't consistent, we note that usage varies. However, it's always a good practice to ensure user convenience, as including a shebang does streamline the execution process.

Supporting multiple Python versions

If your script needs to operate on multiple Python versions, your shebang should not specify a version. Make sure to include a runtime check within it to avoid compatibility issues. In such cases, include a runtime check to manage dependencies and compatibility.

The Shebang Effect

• Direct Execution: Shebang permits scripts to be run directly if marked as executable.
• Manual Invocation: Without shebang, scripts must specifically invoke the Python interpreter.
• Package Tools: Tools like pip, setuptools etc., usually handle shebang wherever necessary.

Shebang's role in portability

The type of shebang can affect your script's portability. For instance, #!/usr/local/bin/python is more restrictive and less cross-platform friendly than the more accommodative #!/usr/bin/env python.

Does the shebang really matter?

In reality, whether you have a shebang or not won't affect the script's execution through the full command (python myscript.py) or when the script is imported as a module.