Why do `s1 == s2` and `s1 is s2` sometimes return different results when comparing strings in Python?

Identity vs. Equality: Unraveling the Comparison Conundrum

In the realm of programming, comparing data can be an essential task. However, when it comes to strings, programmers often encounter unexpected results when using either the '==' or 'is' operators for comparison. Let's delve into this puzzling phenomenon and explore why the output of s1 == s2 and s1 is s2 may differ.

The Dilemma

Consider the following Python code:

s1 = 'text'
s2 = 'text'

Intuitively, one might expect both s1 == s2 and s1 is s2 to return True, as both variables are assigned the same string value. However, while s1 == s2 consistently returns True, signifying equality, s1 is s2 surprisingly returns False in some cases.

Uncovering the Truth

To understand this behavior, it's crucial to differentiate between identity testing (performed by 'is') and equality testing (performed by '=='). Identity testing determines whether two variables refer to the exact same object in memory, while equality testing verifies if their values are identical.

In the Python interpreter, when we assign the same string value to multiple variables, as in the example above, Python optimizes space by storing the string value in a single location in memory and linking all the variables to that location. This means that s1 and s2 refer to the same object and are hence equal in value.

However, Python also implements a mechanism called interning, where certain common string values are stored in a shared pool. When a new string with an interned value is created, Python checks to see if that value already exists in the pool before allocating a new memory location. If it does, the existing location is used, enabling multiple string variables to effectively point to the same memory location.

The Role of Interning

In the case of 'text', it's an interned string value, meaning that both s1 and s2 refer to the same interned value in memory. Consequently, s1 == s2 returns True, confirming their value equality.

However, the reason s1 is s2 sometimes returns False lies in the way Python handles assignments. When we assign a non-interned string value to a variable, a new memory location is allocated for that string. This means that, despite having identical values, the two variables no longer refer to the same object in memory. Hence, s1 is s2 evaluates to False, indicating that they are not the same object.

In Essence

Grasping the distinction between identity testing and equality testing is fundamental in understanding why comparing strings using '==' or 'is' can produce different results. s1 == s2 compares the equality of string values, while s1 is s2 compares the identity of the objects the variables reference in memory.

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