Python Code Translation: From Source to Bytecode

Let's walk through how Python translates your code from plain text to bytecode, step by step. We'll use this example:

a = 6
b = 4
print(a + b)

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1. Source Code (Text)

This is the code you write in a .py file. It's just text, readable by humans.

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2. Tokenization

Python first breaks your code into tokens—the smallest meaningful elements (like names, numbers, and operators).

Example tokens:

Token Type	Token Value
NAME	'a'
OP	'='
NUMBER	'6'
NEWLINE	'\n'
NAME	'b'
OP	'='
NUMBER	'4'
NEWLINE	'\n'
NAME	'print'
OP	'('
NAME	'a'
OP	'+'
NAME	'b'
OP	')'
NEWLINE	'\n'

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3. Parsing and AST (Abstract Syntax Tree)

Next, Python organizes the tokens into a tree structure that represents the meaning of your code. This is called the AST.

AST Outline:

• Module
  • Assign (a = 6)
  • Assign (b = 4)
  • Expr (print(a + b))
    • Call
      • func: print
      • args:
        • BinOp (a + b)

This tree shows how each part of your code relates to the others.

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4. Compilation to Bytecode

Python then compiles the AST into bytecode—a set of instructions for the Python Virtual Machine (PVM).

Example bytecode (using dis module):

  1           0 LOAD_CONST               0 (6)
              2 STORE_NAME               0 (a)
  2           4 LOAD_CONST               1 (4)
              6 STORE_NAME               1 (b)
  3           8 LOAD_NAME                2 (print)
             10 LOAD_NAME                0 (a)
             12 LOAD_NAME                1 (b)
             14 BINARY_OP                0 (+)
             16 PRECALL                  1
             18 CALL                     1
             20 POP_TOP
             22 LOAD_CONST               2 (None)
             24 RETURN_VALUE

Each line is a low-level instruction that the PVM can execute.

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5. Execution

The Python Virtual Machine runs the bytecode, performing the actual calculations and function calls.

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Summary Table

Step	What Happens	Example Output/Structure
Source Code	Human-readable text	a = 6
Tokenization	Break into tokens	NAME, OP, NUMBER, ...
AST	Build tree of code structure	Assign, BinOp, Call, ...
Bytecode	Compile to VM instructions	LOAD_CONST, STORE_NAME, ...
Execution	Python VM runs the bytecode	Output: 10

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