Bytecode

In Python, bytecode is a low-level set of instructions that is portable across different platforms, which means it can be executed on any machine that has a compatible CPython interpreter.

The bytecode runs in the Python virtual machine (PVM) and acts as a bridge between the high-level Python code and the machine code that the CPU executes.

The Python interpreter compiles your source code into bytecode and stores it in .pyc files in the __pycache__/ directory when you import a module. This compiled code allows Python to load modules faster because it skips the compilation step if the bytecode is already up-to-date. Note that bytecode isn’t human-readable and is specific to the Python version that generated it.

Example

Here’s an example that lets you check the bytecode for a function. You can use the dis module to disassemble Python bytecode into a more readable form:

Python 
>>> import dis

>>> def greet(name):
...     return f"Hello, {name}!"
...

>>> dis.dis(greet)
  1           0 RESUME                   0

  2           2 LOAD_CONST               1 ('Hello, ')
              4 LOAD_FAST                0 (name)
              6 FORMAT_VALUE             0
              8 LOAD_CONST               2 ('!')
             10 BUILD_STRING             3
             12 RETURN_VALUE

This output shows the sequence of bytecode instructions that the Python interpreter executes.

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By Leodanis Pozo Ramos • Updated Dec. 19, 2024 • Reviewed by Dan Bader
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