String Concatenation In Python

Can you describe difference between two ways of string concatenation: simple __add__ operator and %s patterns? I had some investigation in this question and found %s (in form witho

Solution 1:

Here is a small exercise:

>>> deff1():
    'hello'>>> deff2():
    'hel''lo'>>> deff3():
    'hel' + 'lo'>>> deff4():
    'hel%s' % 'lo'>>> deff5():
    'hel%s' % ('lo',)


>>> for f in (f1, f2, f3, f4, f5):
    print(f.__name__)
    dis.dis(f)


f1
  10 LOAD_CONST               1 (None) 
              3 RETURN_VALUE         
f2
  10 LOAD_CONST               1 (None) 
              3 RETURN_VALUE         
f3
  20 LOAD_CONST               3 ('hello') 
              3 POP_TOP              
              4 LOAD_CONST               0 (None) 
              7 RETURN_VALUE         
f4
  20 LOAD_CONST               3 ('hello') 
              3 POP_TOP              
              4 LOAD_CONST               0 (None) 
              7 RETURN_VALUE         
f5
  20 LOAD_CONST               1 ('hel%s') 
              3 LOAD_CONST               3 (('lo',)) 
              6 BINARY_MODULO        
              7 POP_TOP              
              8 LOAD_CONST               0 (None) 
             11 RETURN_VALUE         

As you can see, all simple concatenations/formatting are done by compiler. The last function requires more complex formatting and therefore, I guess, is actually executed. Since all those object created at compilation time they all have the same id.

Solution 2:

Using % is, technically speaking, string formatting, not concatenation. They are two entirely* different worlds.

If you know about string interning then you should know that there's absolutely no guarantee that two strings will occupy the same memory as another. The fact that in your example the first three do is nothing more than pure coincidence.

I'm not 100% sure how string formatting works, but I know that it's not implemented the same in the underlying C as concatenation - I think it works a little more along the lines of ''.join(sequence), which is also faster than + for large strings - see this post for more info.

*sort of.