Using eval() function on class methods

Question

I m trying to solve a hackerrank challenge:

Sample Input

S = "qA2"

In the first line, print True if S has any alphanumeric characters. Otherwise, print False.
In the second line, print True if S has any alphabetical characters. Otherwise, print False.
In the third line, print True if S has any digits. Otherwise, print False.
In the fourth line, print True if S has any lowercase characters. Otherwise, print False.
In the fifth line, print True if S has any uppercase characters. Otherwise, print False.

I know there are simpler way to do it but I d like to use eval(). Rather than having multiple line with all the methods, I wanted to use a list with the methods in it (cmd here) then go through it with a for loop, and the eval() should transform it to a method (eval(i+"."+j)). If any of the character i in list(s) returns True, it print true.

Code/Attempt:

if __name__ == '__main__':
 s = input()
 cmd=["isalnum()","isalpha()"]
 for j in cmd:
 print (any([eval(i+"."+j) for i in list(s)]))

Traceback/Error:

Traceback (most recent call last):
 File "Solution.py", line 5, in <module>
 print (any([eval(i+"."+j) for i in list(s)]))
 File "Solution.py", line 5, in <listcomp>
 print (any([eval(i+"."+j) for i in list(s)]))
 File "<string>", line 1, in <module>
NameError: name 'q' is not defined

• juanpa.arrivillaga: Why would you do that? Why use strings and not the functions themselves?
• adriencdw: cauz I want to make the function iterable rather than copy past, I have only 5 methods to apply here. what if I have 100?
• juanpa.arrivillaga: You still don't have to do it this way. That's why I said, use the functions themselves

Answer 1

This is having an issue because when your strings are concatenated in the eval() statement you get something resembling q.isalnum() when you really want 'q'.isalnum(). The first one is using q like a variable when you really want to call the method on the character 'q'. Something like this:

if __name__ == '__main__':
 s = input()
 cmd=["isalnum()","isalpha()"]
 for j in cmd:
 print (any([eval("'{}'.{}".format(i, j)) for i in list(s)]))

Instead of using strings as commands, you could just use the function itself:

cmd=[str.isalnum, str.isalpha]

and then you wouldn't need to use eval as it would just be:

if __name__ == '__main__':
 s = input()
 cmd=[str.isalnum,str.isalpha]
 for j in cmd:
 print (any([j(i) for i in list(s)]))

Edit:

For the sake of completeness, you could also remove the brackets inside the any() to create a generator rather than a list comprehension. This is good because the generator will stop once it first reaches the first True result whereas a list comprehension will evaluate for the whole string. You also don't need to convert s into a list as strings are iterable on their own. You're then left with:

any(j(i) for i in s)

• Jainil Patel: why to do such effort if there is easy way of copy paste 5 times?
• Zach Woods: Because this way is more expandable and dynamic. It may be overkill for the 5 cases that were in the original problem but with this you can reorder the functions and have 100's if not 1000's of different tests without having to change the underlying logic. If you copy paste 1000 times, now you have code that is very hard to manage in the long run because the same thing is repeated over and over.
• adriencdw: that s awesome! super neat code now.how would you explain the str in the str.isalpha? is it correct if I say: 'example'.isalpha() = str.isalpha(example)
• Zach Woods: Yeah, 'example'.isalpha() actually just gets translated to str.isalpha('example'). The first one is just syntactic sugar for the second and in this case str is the class of 'example'. In my example, it was easier to use the str.isalpha because I could then pass the string as a parameter rather than having to build up a function call.
• juanpa.arrivillaga: It's actually not merely syntactic sugar, but the effect is the same.