Unexpected empty strings within Python strings

Question

Observe the following interactive session:

In [1]: s = \'emptiness\'

In [2]: s.replace(\'\', \'*\')
Out[2]: \'*e*m*p*t*i*n*e*s*s*\'

In [3]: s.count(\'\')
O         


        

Answer 1

Python strings follow the principle that an empty string is a subset of every other string. Furthermore, python strings are also concatenations of byte strings, implying that a string consists of bytes sandwiched between empty strings. You can see that by the following examples:

>>>'a'.count('')
2
>>>'aa'.count('')
3
>>>'string'.count('')
7

So 'a' must be ''+'a'+'', and 'aa' must be ''+'a'+''+'a'+''.

When you check 'a'.startswith(''), it sees that the string 'a' technically starts with an empty string. Same for 'a'.endswith(''). However when you check 'a'.startswith('a'), it ignores the empty string and looks at the first byte.