python 3.x - When to use ast.literal_eval

Question

I came across this code and it works, but I am not entirely sure about when to use ast and whether there are performance issues when this is used instead of getting the string value from input() and converting it to int.

import ast

cyper_key = ast.literal_eval(input("Enter the key (a value between 0 and 25) : "))

# this get the user input as an int to the variable cyper_key

I read the docs I understand what it does.

This can be used for safely evaluating strings containing Python values from untrusted sources without the need to parse the values oneself. It is not capable of evaluating arbitrarily complex expressions, for example involving operators or indexing.

I am looking for an explanation on above bold points.

Answer 1

When to use it.

ast.literal_eval(input()) would be useful if you expected a list (or something similar) by the user. For example '[1,2]' would be converted to [1,2].

If the user is supposed to provide a number ast.literal_eval(input()) can be replaced with float(input()), or int(input()) if an integer is expected.

---

Performance

Note that premature [micro-]optimization is the root of all evil. But since you asked:

To test the speed of ast.literal_eval(input()) and float(input() you can use timeit.

Timing will vary based on the input given by the user.

Ints and floats are valid input, while anything else would be invalid. Giving 50% ints, 40% floats and 10% random as input, float(input()) is x12 faster.

With 10%, 10%, 80% and float(input()) is x6 faster.

import timeit as tt

lst_size = 10**5

# Set the percentages of input tried by user.
percentages = {'ints': .10,
               'floats': .10,
               'strings': .80}
assert 1 - sum(percentages.values()) < 0.00000001

ints_floats_strings = {k: int(v*lst_size) for k, v in percentages.items()}

setup = """
import ast

def f(x):
    try:
        float(x)
    except:
        pass

def g(x):
    try:
        ast.literal_eval(x)
    except:
        pass

l = [str(i) for i in range({ints})]
l += [str(float(i)) for i in range({floats})]
l += [']9' for _ in range({strings}//2)] + ['a' for _ in range({strings}//2)]
""".format(**ints_floats_strings)

stmt1 = """
for i in l:
    f(i)
"""

stmt2 = """
for i in l:
    g(i)
"""


reps = 10**1
t1 = tt.timeit(stmt1, setup, number=reps)
t2 = tt.timeit(stmt2, setup, number=reps)

print(t1)
print(t2)

print(t2/t1)