Compare `str` function and format string in Python 3.7

str_and_format_string_in_python37.md

• Benchmark result

root@localhost:~# python3.7 -m timeit -s 'x = 6' 'f"{x}"'
5000000 loops, best of 5: 79 nsec per loop
root@localhost:~# python3.7 -m timeit -s 'x = 6' 'str(x)'
1000000 loops, best of 5: 214 nsec per loop

• Code snippet

def foo(x):
  str(x)

def bar(x):
  f"{x}"

• Bytecode

  2           0 LOAD_GLOBAL              0 (str)
              2 LOAD_FAST                0 (x)
              4 CALL_FUNCTION            1
              6 POP_TOP
              8 LOAD_CONST               0 (None)
             10 RETURN_VALUE
             
  2           0 LOAD_FAST                0 (f)
              2 FORMAT_VALUE             0
              4 POP_TOP
              6 LOAD_CONST               0 (None)
              8 RETURN_VALUE

• CPython

/* Fast path for common types. */
if (format_spec == NULL || PyUnicode_GET_LENGTH(format_spec) == 0) {
    if (PyUnicode_CheckExact(obj)) {
        Py_INCREF(obj);
        return obj;
    }
    if (PyLong_CheckExact(obj)) {
        return PyObject_Str(obj);
    }
}