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# Inspired by the following sentence that I ran across this morning:
#
# "f_lineno is the current line number of the frame - writing to
# this from within a trace function jumps to the given line
# (only for the bottom-most frame). A debugger can implement a
# Jump command (aka Set Next Statement) by writing to f_lineno."
#
# https://docs.python.org/2/reference/datamodel.html
#
# There is an older implementation of a similar idea:
# the 2004 April Fool's joke at
#
# http://entrian.com/goto/index.html
#
# But as their implementation is 110+ lines that re-parses the module
# that uses "goto", I had mistakenly believed that they were editing
# the AST or something to implement goto. Having now re-read their code,
# I see that the parsing is simply to allow forward label references;
# at its core, their logic uses the same f_lineno trick that I am using here.
#
# Oh - I should also note a limitation of this silly experiment, just in
# case it saves time for anyone who plays with it: a goto() is ignored if
# it is the last line of a function, because once a function has completed
# its last line of code the interpreter ignores f_lineno, even if it gets
# pointed back into the body of the function.
import inspect
import sys
from functools import wraps
_python3 = sys.version_info[0] >= 3
def enable_goto(function):
@wraps(function)
def wrapper(*args, **kw):
def trace_call(frame, event, arg):
return trace_line if (frame.f_code is code) else None
def trace_line(frame, event, arg):
if target_list:
frame.f_lineno = target_list.pop()
target_list = []
sys.settrace(trace_call)
try:
function(*args, **kw)
finally:
sys.settrace(None)
code = function.__code__ if _python3 else function.func_code
return wrapper
def goto(target):
inspect.currentframe().f_back.f_back.f_locals['target_list'].append(target)
def label():
return inspect.currentframe().f_back.f_lineno + 1
def offset(n):
return inspect.currentframe().f_back.f_lineno + n
# Goto, without a label:
@enable_goto
def count1():
i = 0
print(i) # this is the line targeted by goto(-3)
i = i + 1
if i < 10:
goto(offset(-3))
print('done')
count1()
# Goto, with a label:
@enable_goto
def count1():
i = 0
top_of_loop = label()
print(i)
i = i + 1
if i < 10:
goto(top_of_loop)
print('done')
count1()
@mauricioabreu

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@mauricioabreu mauricioabreu commented Nov 22, 2014

funny! 😄

@boris-42

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@boris-42 boris-42 commented Nov 22, 2014

Nice!)

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