Python String Comparison: When Should I Use `==` vs. `is`?

Comparing Strings: '==' vs. 'is' Conundrum

In programming, comparing strings is often a crucial operation. However, using the '==' and 'is' operators can sometimes yield different results, leaving developers perplexed.

Understanding the Difference

The '=' operator checks for equality in value, while 'is' checks for identity in memory. Identity testing determines if two variables refer to the exact same object in memory, while equality testing compares their values.

Why the Difference Occurs

When comparing strings, Python optimizes memory usage by reusing existing strings. So, assigning the same string value to multiple variables may not create new string objects but instead refer to the same underlying object. This explains why '==' may return True for equal strings.

However, if the strings are mutated or assigned different values, new objects are created, breaking the identity link. As a result, 'is' would return False even though '==' still returns True because the values are equal.

Example

Consider the following code:

s1 = 'text'
s2 = 'text'

Here, both s1 and s2 reference the same string object, so both '==' and 'is' return True. However, if we modify s2:

s2 = s2 + ' more'

Now, a new string object is created for s2, breaking the identity link. While '==' still returns True because the values are equal, 'is' returns False because s1 and s2 refer to different objects.

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