What is the ast.parse() method in Python?

Overview

In Python, the ast.parse() function splits the source code into tokens based on the grammar. These tokens are then transformed to build an Abstract Syntax Tree (AST). It is a tree representation of a source code. In other words, the ast.parse() method allows the conversion of the source code into a collection of AST nodes linked together as a tree.

Syntax

ast.parse(source, filename='<unknown>', mode='exec', *, type_comments=False, feature_version=None)

Parameters

  • source: The source code in a string, which is to be converted into an AST.
  • filename: The file from which the code is read.
  • mode: The mode in which the code is compiled. It is an exec, which takes a code block that has statements, class, functions, and so on. It can also be an exec that only accepts a single expression.
  • type_comments: The default value is False. If True is returned, the parser is modified to check. It returns type comments specified by PEP 484 , and PEP 526.
  • feature_version: The default value is None. If it is set to a tuple, say (3,4), it attempts to parse using the Python 3.4 version’s grammar.

Return type

The ast.parse() method converts the source code to return a collection of AST nodes.

Code

import ast
from pprint import pprint
tree = ast.parse("""
def add(a, b):
  return a + b
""")
pprint(ast.dump(tree))

Explanation

  • Line 5: We write a function add(a,b) that returns the sum of the input numbers.
  • Line 4: The add function is passed as a string to the ast.parse() method. It transforms the source code into a collection of AST nodes.

The output has several AST nodes, such as:

  • FunctionDef: Function definition.
  • BinOp: Binary operation.

We can also observe arguments inside these nodes. For instance, the value of the Op argument as Add() in the BinOp node.

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