How to use out 'exec()' command in python

Question

For the code below:

command = '\'{:.' + str(3) + 'f}\'.format(' + str(12.6543423) + ')'
print(command)
print('{:.3f}'.format(12.6543423))
print(exec(command))

Expected results:

'{:.3f}'.format(12.6543423)
12.654
12.654

Actual results:

'{:.3f}'.format(12.6543423)
12.654
None

Please can someone tell me what I'm doing wrong and how to fix it? I'm both trying to write a number rounding function and trying to understand the exec command.

• Padraic Cunningham: did you mean eval? have a look here stackoverflow.com/questions/2220699/…
• Ned Batchelder: sigh: there's no need to use exec or eval at all...

Answer 1

Or don't use exec or eval at all. Use the features format offers:

>>> '{:.{}f}'.format(12.6543423, 3)
12.654

• Jake Levi: Ah, thank you - wasn't aware that you could have curly brackets within curly brackets, probably should have tried that first. Thanks. Even so, I'm keeping the current answer ticked, because I was also curious to learn about eval() and exec().
• Ned Batchelder: @JakeLevi learning about eval and exec is good. One of the most important things to learn about them is that they are really really blunt instruments that you almost never need.

Answer 2

Use eval.

>>> eval('\'{:.' + str(3) + 'f}\'.format(' + str(12.6543423) + ')')
'12.654'

• Mauro Baraldi: Use ast.literal_eval instead eval
• Vishnu Upadhyay: @MauroBaraldi Concatenation using + isn't included within that,from the doc The string or node provided may only consist of the following Python literal structures: strings, numbers, tuples, lists, dicts, booleans, and None. and if i am wrong then help me in that. I would love to know.
• sebastian: The + operation is not a "feature" of eval. Python first concatenates the strings and only then passes a single string to eval. That is of course the case with all function calls (in every programming language)...
• Ned Batchelder: @MauroBaraldi: did you try literal_eval? It won't work for this case, because literal_eval won't invoke format for you. It won't call any functions, that's the whole point, to avoid code execution.

Answer 3

Use eval() instead of exec() to get the result returned by a expression.

• Mauro Baraldi: Use ast.literal_eval instead eval
• Ned Batchelder: @MauroBaraldi: did you try literal_eval? It won't work for this case, because literal_eval won't invoke format for you. It won't call any functions, that's the whole point, to avoid code execution.

Answer 4

or you can use exec

command = '\'{:.' + str(3) + 'f}\'.format(' + str(12.6543423) + ')'
print(command)
mycode = """print('{:.3f}'.format(12.6543423))"""
print('{:.3f}'.format(12.6543423))
exec(mycode)

• eivl: it does not. i would also use eval() instead. but it is still usable with exec(). There is in my opinion a lot of confusion regarding exec, also some changes from python 2 and 3.

Answer 5

You are confusing exec and eval:

In [27]: command = 'print'+'('+'\'{:.' + str(3) + 'f}\'.format(' + str(12.6543423) + ')'+')'
In [28]: exec(command)
12.654