print() method to print passed expression literally along with computed output for quick debugging

Question

I wish to be able to perform python debugging using print() or similar method where it prints the passed expression in addition to the usual output.

For instance, for the following code:

print(42 + 42) print(type(list)) print(datetime.now()) 

Current Output:

84 <class 'type'> 2019-08-15 22:43:57.805861 

Expected Output:

42 + 42 : 84 type(list) : <class 'type'> datetime.now() : 2019-08-15 22:43:57.805861 

Currently, the same can be achieved by manually adding the expression string, (not so elegant imho and violates DRY principle).

print("42 + 42 : ", 42 + 42) print("type(list) : ", type(list)) print("datetime.now() : ", datetime.now()) 

I've tried to override builtin print, but without success:

import builtins def print(*args, **kwargs):     return builtins.print(*args, **kwargs)  # passed expression isn't available here as string! 

Is there a way to achieve this? Thanks!

Answer 1

f-strings will support something like this in Python 3.8+.

From the docs:

An f-string such as f'{expr=}' will expand to the text of the expression, an equal sign, then the representation of the evaluated expression. For example:

>>> user = 'eric_idle' >>> member_since = date(1975, 7, 31) >>> f'{user=} {member_since=}' "user='eric_idle' member_since=datetime.date(1975, 7, 31)" 

The usual f-string format specifiers allow more control over how the result of the expression is displayed:

>>> delta = date.today() - member_since >>> f'{user=!s}  {delta.days=:,d}' 'user=eric_idle  delta.days=16,075' 

The = specifier will display the whole expression so that calculations can be shown:

>>> print(f'{theta=}  {cos(radians(theta))=:.3f}') theta=30  cos(radians(theta))=0.866 

Answer 2

Generally I think if you find yourself using eval there's probably a better way to do what you're trying to do, but:

for statement in ["42 + 42", "type(list)", "datetime.now()"]:     print("{} : {}".format(statement, eval(statement))