104 – AST parsing

The module ast has a function parse that you can use to parse code (in a string) into what’s called an AST. An AST is an “Abstract Syntax Tree”, an intermediate representation of your code that Python operates on.

Studying the ASTs produced by the module ast can teach you a lot about the Python language. For example, by using ast.parse you can find out that the keyword elif is just syntactic sugar for a sequence of else: if ...: at the language level:

import ast

code = """\
if a:
    pass
else:         # <--
    if b:     # <--
        pass
    else:
        pass
"""

print(ast.dump(
    ast.parse(code),
    indent=2,
))

Module(
  body=[
    If(
      test=Name(id='a', ctx=Load()),
      body=[
        Pass()],
      orelse=[                            <--
        If(                               <--
          test=Name(id='b', ctx=Load()),
          body=[
            Pass()],
          orelse=[
            Pass()])])])

This structure is identical to the one you get if you use elif instead of else: if ...::

import ast

code = """\
if a:
    pass
elif b:   # <--
    pass
else:
    pass
"""

print(ast.dump(
    ast.parse(code),
    indent=2,
))  # Exact same output.