Why does comparing strings using either '==' or 'is' sometimes produce a different result?

When comparing strings using the == operator, the comparison is based on the actual characters in the string. For example:

Comparing strings using the '==' operator in Python

string1 = "Hello"
string2 = "Hello"
string3 = "hello"

print(string1 == string2) # True
print(string1 == string3) # False

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On the other hand, the is operator checks for object identity, meaning it compares memory addresses. If two strings are identical in content but stored in different memory locations, is will return False. However, Python automatically interns certain strings (like short literals), which means identical string literals may share the same memory address and is will return True.

For example:

Comparing strings using the 'is' keyword in Python

string1 = "Hello"
string2 = "Hello"
string3 = "".join(["H", "e", "l", "l", "o"])

print(string1 is string2) # True
print(string1 is string3) # False

In this example, string1 and string2 are identical string literals, so Python interns them and they share the same memory address, making string1 is string2 evaluate to True. Meanwhile, string3 is created dynamically using "".join(), so it resides in a different memory location. Therefore, string1 is string3 evaluates to False.

Because is behavior depends on implementation details like interning rather than string content, you should generally use == for comparing string values.