Its a big confusion for people who just started working in python and the answers here are a little difficult to comprehend so i'll make it easier.
When we instruct Python to run our script, there are a few steps that Python carries out before our code actually starts crunching away:
- It is compiled to bytecode.
- Then it is routed to virtual machine.
When we execute some source code, Python compiles it into byte code. Compilation is a translation step, and the byte code is a low-level platform-independent representation of source code.
Note that the Python byte code is not binary machine code (e.g., instructions for an Intel chip).
Actually, Python translate each statement of the source code into byte code instructions by decomposing them into individual steps. The byte code translation is performed to speed execution.
Byte code can be run much more quickly than the original source code statements. It has.pyc extension and it will be written if it can write to our machine.
So, next time we run the same program, Python will load the .pyc file and skip the compilation step unless it's been changed. Python automatically checks the timestamps of source and byte code files to know when it must recompile. If we resave the source code, byte code is automatically created again the next time the program is run.
If Python cannot write the byte code files to our machine, our program still works. The byte code is generated in memory and simply discarded on program exit. But because .pyc files speed startup time, we may want to make sure it has been written for larger programs.
Let's summarize what happens behind the scenes.
When Python executes a program, Python reads the .py into memory, and parses it in order to get a bytecode, then goes on to execute. For each module that is imported by the program, Python first checks to see whether there is a precompiled bytecode version, in a .pyo or .pyc, that has a timestamp which corresponds to its .py file. Python uses the bytecode version if any. Otherwise, it parses the module's .py file, saves it into a .pyc file, and uses the bytecode it just created.
Byte code files are also one way of shipping Python codes. Python will still run a program if all it can find are.pyc files, even if the original .py source files are not there.
Python Virtual Machine (PVM)
Once our program has been compiled into byte code, it is shipped off for execution to Python Virtual Machine (PVM). The PVM is not a separate program. It need not be installed by itself. Actually, the PVM is just a big loop that iterates through our byte code instruction, 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. It's the component that truly runs our scripts. Technically it's just the last step of what is called the Python interpreter.