goto

A function decorator to use goto in Python. Tested on Python 2.6 through 3.7 and PyPy.

Installation

pip install goto-statement

Usage

from goto import with_goto

@with_goto
def range(start, stop):
    i = start
    result = []

    label .begin
    if i == stop:
        goto .end

    result.append(i)
    i += 1
    goto .begin

    label .end
    return result

Implementation

Note that label .begin and goto .begin is regular Python syntax to retrieve the attribute begin from the objects with the variable names label and goto. However, in the example above these variables aren't defined. So this code would usually cause a NameError. But since it's valid syntax the function can be parsed, and results in following bytecode:

  5           0 LOAD_FAST                0 (start)
              2 STORE_FAST               2 (i)

  6           4 BUILD_LIST               0
              6 STORE_FAST               3 (result)

  8           8 LOAD_GLOBAL              0 (label)
             10 LOAD_ATTR                1 (begin)
             12 POP_TOP

  9          14 LOAD_FAST                2 (i)
             16 LOAD_FAST                1 (stop)
             18 COMPARE_OP               2 (==)
             20 POP_JUMP_IF_FALSE       28

 10          22 LOAD_GLOBAL              2 (goto)
             24 LOAD_ATTR                3 (end)
             26 POP_TOP

 12     >>   28 LOAD_FAST                3 (result)
             30 LOAD_METHOD              4 (append)
             32 LOAD_FAST                2 (i)
             34 CALL_METHOD              1
             36 POP_TOP

 13          38 LOAD_FAST                2 (i)
             40 LOAD_CONST               1 (1)
             42 INPLACE_ADD
             44 STORE_FAST               2 (i)

 14          46 LOAD_GLOBAL              2 (goto)
             48 LOAD_ATTR                1 (begin)
             50 POP_TOP

 16          52 LOAD_GLOBAL              0 (label)
             54 LOAD_ATTR                3 (end)
             56 POP_TOP

 17          58 LOAD_FAST                3 (result)
             60 RETURN_VALUE

The with_goto decorator then removes the respective bytecode that has been generated for the attribute lookups of the label and goto variables, and injects a JUMP_ABSOLUTE or JUMP_RELATIVE instruction for each goto:

  5           0 LOAD_FAST                0 (start)
              2 STORE_FAST               2 (i)

  6           4 BUILD_LIST               0
              6 STORE_FAST               3 (result)

  8           8 NOP
             10 NOP
             12 NOP

  9     >>   14 LOAD_FAST                2 (i)
             16 LOAD_FAST                1 (stop)
             18 COMPARE_OP               2 (==)
             20 POP_JUMP_IF_FALSE       28

 10          22 JUMP_FORWARD            34 (to 58)
             24 NOP
             26 NOP

 12     >>   28 LOAD_FAST                3 (result)
             30 LOAD_METHOD              4 (append)
             32 LOAD_FAST                2 (i)
             34 CALL_METHOD              1
             36 POP_TOP

 13          38 LOAD_FAST                2 (i)
             40 LOAD_CONST               1 (1)
             42 INPLACE_ADD
             44 STORE_FAST               2 (i)

 14          46 JUMP_ABSOLUTE           14
             48 NOP
             50 NOP

 16          52 NOP
             54 NOP
             56 NOP

 17     >>   58 LOAD_FAST                3 (result)
             60 RETURN_VALUE

Alternative implementation

The idea of goto in Python isn't new. There is another module that has been released as April Fool's joke in 2004. That implementation doesn't touch the bytecode, but uses a trace function, similar to how debuggers are written.

While this eliminates the need for a decorator, it comes with significant runtime overhead and a more elaborate implementation. Modifying the bytecode, on the other hand, is fairly simple and doesn't add overhead at function execution.