Why does Python not perform type conversion when concatenating strings?

Question

In Python, the following code produces an error:

a = 'abc' b = 1 print(a + b) 

(The error is "TypeError: cannot concatenate 'str' and 'int' objects").

Why does the Python interpreter not automatically try using the str() function when it encounters concatenation of these types?

Answer 1

The problem is that the conversion is ambiguous, because + means both string concatenation and numeric addition. The following question would be equally valid:

Why does the Python interpreter not automatically try using the int() function when it encounters addition of these types?

This is exactly the loose-typing problem that unfortunately afflicts Javascript.

Answer 2

There's a very large degree of ambiguity with such operations. Suppose that case instead:

a = '4' b = 1 print(a + b) 

It's not clear if a should be coerced to an integer (resulting in 5), or if b should be coerced to a string (resulting in '41'). Since type juggling rules are transitive, passing a numeric string to a function expecting numbers could get you in trouble, especially since almost all arithmetic operators have overloaded operations for strings too.

For instance, in Javascript, to make sure you deal with integers and not strings, a common practice is to multiply a variable by one; in Python, the multiplication operator repeats strings, so '41' * 1 is a no-op. It's probably better to just ask the developer to clarify.