How Does Python String Interning Affect String Comparisons and When Should You Use `sys.intern()`?

Understanding Python String Interning

String interning, a Python implementation detail, involves creating only one instance of a string value when it occurs multiple times in a program. This optimization speeds up program execution by reducing the memory used for string storage.

In Python, compile-time constant strings are typically interned. As a result, you can compare two string literals directly for equality:

"string" is "string"

This comparison returns True because the two string literals are the same object.

However, string interning does not apply to runtime expressions. Consider the following code:

s1 = "strin"
s2 = "string"
s1 + "g" is s2

You might expect this expression to evaluate to True since adding "g" to "strin" results in "string." However, it returns False. This is because the concatenation operation does not create an interned string:

s3a = "strin"
s3 = s3a + "g"
s3 is "string"

In this case, s3 is a new string object, distinct from both s3a and "string."

You can manually intern a string using the sys.intern() function:

sys.intern(s3) is "string"

This line of code forces Python to create an interned string with the same value as s3. As a result, the comparison to "string" returns True.

In conclusion, Python string interning applies to compile-time constants. However, runtime string operations such as concatenation do not automatically create interned strings. By understanding this implementation detail, you can optimize code performance by carefully using string interning.

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