Python palindrome checker function

Question

For practice purposes I implemented a function that checks for palindromes:

def palindrom(name):
 name_reversed = ""
 for buchstabe in name[::-1]:
 name_reversed += buchstabe
 return name.lower() == name_reversed.lower()
print(palindrom("Tom"))
print(palindrom("Bob"))

How may I improve this code, especially the for-loop?

• Reinderien: Great first question!
• Vishesh Mangla: btw what's a buchstabe?
• In Hoc Signo: @VisheshMangla It's German for "letter".
• Vishesh Mangla: Thank's @TheDaleks

Answer 1

Localisation

It's fine to localise your user-facing strings (e.g. print('Die Buchstaben in diesem Wort bilden ein Palindrom'), but it is not advisable to write the code itself in a language other than English. For better or worse, English is the de-facto language of programming. Thus, buchstabe would be better as letter.

For-loop

The loop is not necessary:

 name_reversed = name[::-1]

name is a string, which is itself a sequence of one-character strings. Applying a slice, as is done here, reverses the sequence but does not change the type: the output of the expression is still a string.

• DeepSpace: Not only that "the loop is not necessary", creating an empty string and then concatenating to it in a loop is an anti-pattern in Python since it has a very poor performance.

Answer 2

For your for-loop, you can use reversed(name) instead of name[::-1]. This will increase performance by preventing unnecessary copying by creating a generator instead of another string. However in your case, because you are only checking if the reversed is the same as the original, your function can be even simpler

def palindrome(name):
 return name.lower() == name[::-1].lower()

• user7761803: This lowercases two strings - why not lowercase the string on input (i.e. just once) then do the reverse and compare?
• QuantumChris: Ideally when comparing strings you would use .casefold() instead of .lower() since it handles cases such as ß -> ss.
• vlovero: @QuantumChris yes you are correct, I left it as .lower() because I wanted it to look simple (not necessarily perfect) and from what I've seen, no one seems to know about .casefold()

Answer 3

It's also possible to work with two pointers. This is not the shortest solution, but it should be very fast and you don't have to store the reversed string.

def palindrome(name):
 name = name.lower()
 left = 0
 right = len(name) - 1
 while left < right:
 if name[left] != name[right]:
 return False
 left += 1
 right -= 1
 return True

• Reinderien: This is Python - there are no pointers.
• Reinderien: Also, this will not be fast. I encourage you to compare it with timeit.

Answer 4

@Reinderien and @vlovero have already told whatever could be done but maybe you would also like the reversed function. Also you do not need to lower() your strings twice when you can do it only once. Strings(immutable type) are passed by value in python and doesn't modify the original string since a shallow copy of your string is sent to the function.

def palindrome(name):
 name = name.lower()
 return "".join(reversed(name)) == name #or simply name[::-1] == name
print(palindrome("Tom"))
print(palindrome("Bob"))

• Rob: This also points to another python perfomance issue, you rarely want to do repeated += with strings as strings are immutable so you end up making a copy of the intermediate string each time. Instead if you need to build a string up, add each element to a list then do "".join() at the end.
• Vishesh Mangla: Yep, you are right.
• DeepSpace: This beats every advantage that reversed brings to the table (ie the fact it returns an iterator rather than a string). Just use [::-1] and get back a reversed string directly instead of needing to use ''.join.
• Vishesh Mangla: I 'm not sure how [::-1] works. I can't say anything. When I learned C++ 3-4 years ago I was told strings take 1 byte(256 ascii) and they require contiguous memory and thus they are immutable since if you break the pattern of the string like swap 3rd and 4th character the addresses order ruins. Since python's too implemented in c++ I do believe this slicing wouldn't be as simple as you thing it to be.