![importinginpython
# very simple version
def import_module(name):
if name in sys.modules:
return sys.modules[name]
module = internal_import(name)
sys.modules[name] = module
return module](https://image.slidesharecdn.com/europython2016shashkova-160722141517/85/Monkey-patching-in-Python-a-magic-trick-or-a-powerful-tool-38-320.jpg)
![savemodules
saved_modules.py:
import socket
# save the module object
save = sys.modules.pop('socket', None)
# it's not in sys.modules, reimport
import socket as socket_saved
# put the module available for patching
sys.modules['socket'] = save](https://image.slidesharecdn.com/europython2016shashkova-160722141517/85/Monkey-patching-in-Python-a-magic-trick-or-a-powerful-tool-39-320.jpg)
Monkey-patching in Python: a magic trick or a powerful tool?
- 2. bio • Software Developer of PyCharm IDE at JetBrains • Debugger used in PyDev & PyCharm 2
- 3. Monkey-patching is a dynamic modification of a class or a module at runtime def safe_sqrt(num): # doesn’t throw exception if num < 0 if num < 0: return math.nan return math.original(num) >>> import math >>> math.original = math.sqrt >>> math.sqrt = safe_sqrt monkey-patching
- 4. monkey-patching • Guerilla patch • Does something secretly and incompatibly with others • Gorilla patch • Monkey patch
- 5. otherlanguages Ruby class String def upcase self.reverse end end > puts "hello".upcase "olleh"
- 6. python Explicit is better than implicit
- 8. whatcanwepatch? Almost Everything! We can’t patch built-ins defined in C
- 10. •Workaround for bugs in third-party code lightside
- 11. •Workaround for bugs in third-party code • Don’t forget to fix and create a pull request! lightside
- 12. •Workaround for bugs in third-party code • Don’t forget to fix and create a pull request! •Code instrumentation lightside
- 13. •Workaround for bugs in third-party code • Don’t forget to fix and create a pull request! •Code instrumentation •Testing lightside
- 14. •Workaround for bugs in third-party code • Don’t forget to fix and create a pull request! •Code instrumentation •Testing •Changing of standard modules lightside
- 15. examples
- 17. lightside:newprocess class ProcessManager: def __init__(self, system_fork): self.system_fork = system_fork def do_fork(self): pid = self.system_fork() if pid == 0: # child process start_tracing() return pid
- 18. import os manager = ProcessManager(os.fork) os.fork = manager.do_fork os.fork() #tracing in a new process lightside:newprocess
- 20. •How to patch a module as soon as possible? •Patch it right after importing lightside
- 21. lightside:importhook class ImportHookManager: def __init__(self, system_import): self.system_import = system_import def do_import(self, name): module = self.system_import(name) if name == "os": orig_fork = module.fork module.fork = ProcessManager(orig_fork).do_fork return module
- 22. lightside:importhook import builtins manager = ImportHookManager(builtins.__import__) builtins.__import__ = manager.do_import import os # module is patched as soon as it is imported
- 23. lightside:gevent
- 24. lightside:gevent ● gevent provides asynchronous I/O ● Blocking system calls in the standard library import socket ● Replace with gevent modules from gevent import socket
- 25. lightside:gevent ! The simplest way from gevent import monkey monkey.patch_socket()
- 26. •Workaround for bugs in third-party code •Code instrumentation •Testing •Changing of standard modules lightside
- 30. darkside • Implicit changes • Undocumented changes
- 31. darkside • Implicit changes • Undocumented changes • Unpredictable behaviour
- 32. darkside • Implicit changes • Undocumented changes • Unpredictable behaviour • Patching the same
- 33. examples
- 34. darkside:gevent
- 35. darkside:gevent • Patches standard modules socket, thread, time and others • Should be done as soon as possible
- 36. runningcodeunderdebugger • Event loop in debugger Based on threads • Event loop in user’s code Based on gevent loop They should be separated
- 37. theproblem Save the original versions of modules and use them with the patched versions
- 38. importinginpython # very simple version def import_module(name): if name in sys.modules: return sys.modules[name] module = internal_import(name) sys.modules[name] = module return module
- 39. savemodules saved_modules.py: import socket # save the module object save = sys.modules.pop('socket', None) # it's not in sys.modules, reimport import socket as socket_saved # put the module available for patching sys.modules['socket'] = save
- 40. savemodules from saved_modules import saved_socket import socket # it will patch socket in sys.path gevent.patch_socket() # the patched function socket.create_connection() # the original function saved_socket.create_connection()
- 41. theproblem Save the original versions of modules and use them with the patched versions Solved!
- 42. ● Implicit changes ● Undocumented changes ● Broken readability ● Incompatibility with other patches ● Workaround for bugs in third-party code ● Code instrumentation ● Testing ● Changing of standard modules lightsideof monkey-patching darksideof monkey-patching
- 43. whenshouldiuseit? If and only if there are no other solutions for the problem