Python sympy Evaluating When Asked Not To

Question

This works as expected:

>>> from sympy.parsing.sympy_parser import parse_expr 
>>> parse_expr("2**3"), parse_expr("2**3", evaluate=False)
(8, 2**3)

This, however, not:

>>> from sympy.parsing.sympy_parser import parse_expr
>>> parse_expr("sqrt(9)"), parse_expr("sqrt(9)", evaluate=False)
(3, 3)

I would expect:

(3, sqrt(9))

Any ideas, why?

Answer 1

The evaluate flag to parse_expr only affects the direct evaluation of the expression. sqrt(x) is short for x**Rational(1, 2), which isn't part of the expression parsing.

You can use the with evaluate(False) decorator to prevent the power in the sqrt function from evaluating:

>>> with evaluate(False):
...     print(parse_expr('sqrt(9)', evaluate=False))
sqrt(9)
>>>

(I kept the evaluate=False flag, but it's probably not actually needed)