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ast.Expression

root and base nodes

The root of the tree built by ast.parse(src, mode='eval'): a single expression in the body field, ready to be compiled for eval().

Syntax

ast.parse(source, mode='eval') -> ast.Expression(body)

Unlike Module, the body of an Expression is a single node, not a list: tree.body[0] fails with TypeError here. The 'eval' mode rejects statements outright: the string 'x = 1' raises SyntaxError while parsing. Do not confuse it with ast.Expr, the expression statement inside Module.body (such as a bare print(...) call). An Expression tree must be compiled with mode='eval', otherwise compile() raises TypeError.

Examples

import ast
tree = ast.parse('2 * 21', mode='eval')
print(type(tree).__name__, type(tree.body).__name__)  # → Expression BinOp
print(eval(compile(tree, '<expr>', 'eval')))  # → 42

import ast
try:
    ast.parse('x = 1', mode='eval')
except SyntaxError:
    print('mode=eval: only expressions')  # → mode=eval: only expressions

See also

Common errors

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