Class AssertionRewriter

src/_pytest/assertion/rewrite.py:606–1154

Assertion rewriting implementation. The main entrypoint is to call .run() with an ast.Module instance, this will then find all the assert statements and rewrite them to provide intermediate values and a detailed assertion error. See http://pybites.blogspot.be/2011/07/behind-scenes-

class AssertionRewriter(ast.NodeVisitor):
    """Assertion rewriting implementation.

    The main entrypoint is to call .run() with an ast.Module instance,
    this will then find all the assert statements and rewrite them to
    provide intermediate values and a detailed assertion error.  See
    http://pybites.blogspot.be/2011/07/behind-scenes-of-pytests-new-assertion.html
    for an overview of how this works.

    The entry point here is .run() which will iterate over all the
    statements in an ast.Module and for each ast.Assert statement it
    finds call .visit() with it.  Then .visit_Assert() takes over and
    is responsible for creating new ast statements to replace the
    original assert statement: it rewrites the test of an assertion
    to provide intermediate values and replace it with an if statement
    which raises an assertion error with a detailed explanation in
    case the expression is false and calls pytest_assertion_pass hook
    if expression is true.

    For this .visit_Assert() uses the visitor pattern to visit all the
    AST nodes of the ast.Assert.test field, each visit call returning
    an AST node and the corresponding explanation string.  During this
    state is kept in several instance attributes:

    :statements: All the AST statements which will replace the assert
       statement.

    :variables: This is populated by .variable() with each variable
       used by the statements so that they can all be set to None at
       the end of the statements.

    :variable_counter: Counter to create new unique variables needed
       by statements.  Variables are created using .variable() and
       have the form of "@py_assert0".

    :expl_stmts: The AST statements which will be executed to get
       data from the assertion.  This is the code which will construct
       the detailed assertion message that is used in the AssertionError
       or for the pytest_assertion_pass hook.

    :explanation_specifiers: A dict filled by .explanation_param()
       with %-formatting placeholders and their corresponding
       expressions to use in the building of an assertion message.
       This is used by .pop_format_context() to build a message.

    :stack: A stack of the explanation_specifiers dicts maintained by
       .push_format_context() and .pop_format_context() which allows
       to build another %-formatted string while already building one.

    :scope: A tuple containing the current scope used for variables_overwrite.

    :variables_overwrite: A dict filled with references to variables
       that change value within an assert. This happens when a variable is
       reassigned with the walrus operator

    This state, except the variables_overwrite,  is reset on every new assert
    statement visited and used by the other visitors.
    """