# 19.3 Python Abstract Syntax

The `compiler.ast` module defines an abstract syntax for Python. In the abstract syntax tree, each node represents a syntactic construct. The root of the tree is `Module` object.

The abstract syntax offers a higher level interface to parsed Python source code. The `parser` module and the compiler written in C for the Python interpreter use a concrete syntax tree. The concrete syntax is tied closely to the grammar description used for the Python parser. Instead of a single node for a construct, there are often several levels of nested nodes that are introduced by Python's precedence rules.

The abstract syntax tree is created by the `compiler.transformer` module. The transformer relies on the builtin Python parser to generate a concrete syntax tree. It generates an abstract syntax tree from the concrete tree.

The `transformer` module was created by Greg Stein and Bill Tutt for an experimental Python-to-C compiler. The current version contains a number of modifications and improvements, but the basic form of the abstract syntax and of the transformer are due to Stein and Tutt.