Is it ever appropriate to join two strings using the plus sign (+) over concatenating with curly brackets ({}) and `format` in Python 2.7?

Question

I'm trying to write clean and concise code, and in a lot of code I look over sometimes I see people are inconsistent in their code. What I'm asking is, is there ever an instance where this

print("Cars on the road: " + cars)

is more appropriate than this

print("Cars on the road: {}".format(cars))

or is it just a matter of preference?

Answer 1

The big functional difference between the two examples you gave is that when you concatenate with +, the operation will fail if the object on the right side of the operand is not a string:

"abc" + object()

For instance will cause the following:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't convert 'object' object to str implicitly

This is true even if the object on the right side implements the __str__ method:

class Foo:
    def __str__(self):
        return "str"

Using format however will automatically convert a passed argument using the __str__ method:

"{}".format(Foo()) # "str"

There are some situations where this behavior might not be desirable or necessary, such as when you are simply concatenating a string literal with an object that is expected to be a string.

In all other cases I agree with the post cited in the comments which provide plenty of good reasons why formatting is more idiomatically correct and potentially more efficient.