has_equal_ast

has_equal_ast(incorrect_msg='FMT: Your code does not seem to match the solution.', code=None, exact=True, state=None)

Test whether abstract syntax trees match between the student and solution code.

Parameters:

• incorrect_msg – message displayed when ASTs mismatch.
• code – optional code to use instead of the solution AST
• exact – whether the representations must match exactly. If false, the solution AST only needs to be contained within the student AST (similar to using test student typed).

Example:

Student and Solution Code:

dict(a = 'value').keys()

SCT:

# all pass
Ex().has_equal_ast()
Ex().has_equal_ast(code = "dict(a = 'value').keys()")
Ex().has_equal_ast(code = "dict(a = 'value')", exact = False)

An abstract syntax tree (AST) is a way of representing the high-level structure of python code.

Example: quotes

Whether you use the concrete syntax x = "1" or x = '1', the abstract syntax is the same: x is being assigned to the string “1”.

Example: parenthesis

Grouping by parentheses produces the same AST, when the same statement would work the same without them. For example, (True or False) and True, and True or False and True, are the same due to operator precedence.

Example: spacing

The same holds for different types of spacing that essentially specify the same statement: x = 1 or x = 1.

Caveat: evaluating

What the AST doesn’t represent is values that are found through evaluation. For example, the first item in the list in

x = 1
[x, 2, 3]

and

[1, 2, 3]

Is not the same. In the first case, the AST represents that a variable x needs to be evaluated in order to find out what its value is. In the second case, it just represents the value 1.