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What does python do when you ask it to run a script ?

Updated: Sep 1, 2020




Although knowledge of Python internals is not strictly required for Python programming, a basic understanding of the runtime structure of Python can help you grasp the bigger picture of program execution.

When you instruct Python to run your script, there are a few steps that Python carries out before your code actually starts crunching away. Specifically, it’s first compiled to something called “byte code” and then routed to something called a “virtual machine.” Lets understand these in detail.


Byte code compilation

Internally, when you execute a program Python first compiles your source code (the statements in your file) into a format known as byte code. Compilation is simply a translation step, and byte code is a lower-level, platform-independent representation of your source code. Roughly, Python translates each of your source statements into a group of byte code instructions by decomposing them into individual steps. This byte code translation is performed to speed execution —byte code can be run much more quickly than the original source code statements in your text file.


If the Python process has write access on your machine, it will store the byte code of your programs in files that end with a .pyc extension (“.pyc” means compiled “.py” source). Prior to Python 3.2, you will see these files show up on your computer after you’ve run a few programs alongside the corresponding source code files—that is, in the same directories. For instance, you’ll notice a script.pyc after importing a script.py.

In 3.2 and later, Python instead saves its .pyc byte code files in a subdirectory named __pycache__ located in the directory where your source files reside, and in files whose names identify the Python version that created them (e.g., script.cpython-33.pyc). The new __pycache__ subdirectory helps to avoid clutter, and the new naming convention for byte code files prevents different Python versions installed on the same computer from overwriting each other’s saved byte code.


In both models, Python saves byte code like this as a startup speed optimization. The next time you run your program, Python will load the .pyc files and skip the compilation step, as long as you haven’t changed your source code since the byte code was last saved, and aren’t running with a different Python than the one that created the byte code. It works like this:


  1. Source changes: Python automatically checks the last-modified timestamps of source and byte code files to know when it must recompile—if you edit and resave your source code, byte code is automatically re-created the next time your program is run.

  2. Python versions: Imports also check to see if the file must be recompiled because it was created by a different Python version, using either a “magic” version number in the byte code file itself in 3.2 and earlier, or the information present in byte code filenames in 3.2 and later.


The result is that both source code changes and differing Python version numbers will trigger a new byte code file. If Python cannot write the byte code files to your machine, your program still works—the byte code is generated in memory and simply discarded on program exit. However, because .pyc files speed startup time, you’ll want to make sure they are written for larger programs. Byte code files are also one way to ship Python programs—Python is happy to run a program if all it can find are .pyc files, even if the original .py source files are absent. Byte code is never saved for code typed at the interactive prompt—a programming mode


The Python Virtual Machine (PVM)

Once your program has been compiled to byte code (or the byte code has been loaded from existing .pyc files), it is shipped off for execution to something generally known as the Python Virtual Machine (PVM). The PVM sounds more impressive than it is; really, it’s not a separate program, and it need not be installed by itself. In fact, the PVM is just a big code loop that iterates through your byte code instructions, one by one, to carry out their operations. The PVM is the runtime engine of Python; it’s always present as part of the Python system, and it’s the component that truly runs your scripts. Technically, it’s just the last step of what is called the “Python interpreter.

Keep in mind that all of this complexity is deliberately hidden from Python programmers. Byte code compilation is automatic, and the PVM is just part of the Python system that you have installed on your machine. Again, programmers simply code and run files of statements, and Python handles the logistics of running them.


Frozen Binaries

It is possible to turn your Python programs into true executables, known as frozen binaries in the Python world. These programs can be run without requiring a Python installation.


Frozen binaries bundle together the byte code of your program files, along with the PVM (interpreter) and any Python support files your program needs, into a single package. There are some variations on this theme, but the end result can be a single binary executable program (e.g., an .exe file on Windows) that can easily be shipped to users.


Frozen binaries are not the same as the output of a true compiler—they run byte code through a virtual machine. Hence, apart from a possible startup improvement, frozen binaries run at the same speed as the original source files. Frozen binaries are also not generally small (they contain a PVM), but by current standards they are not unusually large either. Because Python is embedded in the frozen binary, though, it does not have to be installed on the receiving end to run your program. Moreover, because your code is embedded in the frozen binary, it is more effectively hidden from recipients.


Once you are clear on this - you might want to understand how are objects placed conceptually in python

Python Conceptual Hierarchy

and, to conceptually understand how python creates objects read this


How does python store objects and variables.


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Happy learning !!


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