Why were literal formatted strings (f-strings) so slow in Python 3.6 alpha? (now fixed in 3.6 stable)

Note: This answer was written for the Python 3.6 alpha releases. A new opcode added to 3.6.0b1 improved f-string performance significantly.

The f"..." syntax is effectively converted to a str.join() operation on the literal string parts around the {...} expressions, and the results of the expressions themselves passed through the object.__format__() method (passing any :.. format specification in). You can see this when disassembling:

>>> import dis
>>> dis.dis(compile('f"X is {x}"', '', 'exec'))
  1           0 LOAD_CONST               0 ('')
              3 LOAD_ATTR                0 (join)
              6 LOAD_CONST               1 ('X is ')
              9 LOAD_NAME                1 (x)
             12 FORMAT_VALUE             0
             15 BUILD_LIST               2
             18 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             21 POP_TOP
             22 LOAD_CONST               2 (None)
             25 RETURN_VALUE
>>> dis.dis(compile('"X is {}".format(x)', '', 'exec'))
  1           0 LOAD_CONST               0 ('X is {}')
              3 LOAD_ATTR                0 (format)
              6 LOAD_NAME                1 (x)
              9 CALL_FUNCTION            1 (1 positional, 0 keyword pair)
             12 POP_TOP
             13 LOAD_CONST               1 (None)
             16 RETURN_VALUE

Note the BUILD_LIST and LOAD_ATTR .. (join) op-codes in that result. The new FORMAT_VALUE takes the top of the stack plus a format value (parsed out at compile time) to combine these in a object.__format__() call.

So your example, f"X is {x}", is translated to:

''.join(["X is ", x.__format__('')])

Note that this requires Python to create a list object, and call the str.join() method.

The str.format() call is also a method call, and after parsing there is still a call to x.__format__('') involved, but crucially, there is no list creation involved here. It is this difference that makes the str.format() method faster.

Note that Python 3.6 has only been released as an alpha build; this implementation can still easily change. See PEP 494 – Python 3.6 Release Schedule for the time table, as well as Python issue #27078 (opened in response to this question) for a discussion on how to further improve the performance of formatted string literals.