5 Years of Bad Ideas

Armin Ronacher

Introduction

What defines Python?

Core Values

Beautiful Code Low-clutter Syntax Everything is a rst class object, functions included

But Also

Ability to eval() and compile() Access to interpreter internals A community that can monkey patch, but would not do it unless necessary.

Good or Bad Magic

About Magic
And honestly, I only nd this shit in web frameworks. WTF is up with Python people going ballistic when a web app uses magic? Bullshit.
Zed Shaw about Magic

Magic as Enabler
Certain things require magic (Having an interactive traceback in your browser when a web application fails) Stepping through code with a debugger Properly post-processed traceback frames for generated code. Generated code in general.

Magic with Fallbacks

Magic is ne for as long as there is a fallback. Magic becomes a burden if it's the only way to achieve something in production code.

Common Magic

sys._getframe

import sys

def caller_lineno(): return sys._getframe(1).f_lineno def caller_filename(): return sys._getframe(1).f_code.co_filename

class User(db.Model): implements(IRenderable) ... def render(self, renderer): renderer.text(self.username) renderer.img(src=self.icon_avatar_url, class_='small-avatar')

import sys

def implements(*interfaces): cls_scope = sys._getframe(1).f_locals metacls = cls_scope.get('__metaclass__') new_metacls = magic_metaclass_factory( interfaces, metacls) cls_scope['__metaclass__'] = new_metaclas

import sys

def find_request(): frm = sys._getframe(1) while frm is not None: if 'request' in frm.f_locals and \ hasattr(frm.f_locals['request'], 'META'): return frm.f_locals['request'] frm = frm.f_back

Metaclasses

class AwesomeRegistry(type): registry = {} def __new__(cls, name, bases, d): rv = type.__new__(cls, name, bases, d) registry[name] = rv return rv def __getitem__(cls, name): return cls.registry[name]

class AwesomeBase(object): __metaclass__ = registry

exec / compile

def func_from_file(filename, function): namespace = {} execfile(filename, namespace) return namespace[function] func = func_from_file('hello.cfg', 'main') func()

AST Hacks

>>> >>> >>> >>> -1

import ast x = ast.parse('1 + 2', mode='eval') x.body.op = ast.Sub() eval(compile(x, '<string>', 'eval'))

Things you can do

Coupled with an import hook, rewrite assert statements to method calls Implement macros Use it for code generation

@macro def add(a, b): return a + b

@macro def times(i): for __x in range(i): __body__

def doing_something(): a = add(1, 2) with times(10): print 'iterating ...'

Import Hooks

>>> from githubimporter import GitHubImporter >>> sys.path_hooks.append(GitHubImporter) >>> import sys >>> sys.path.append('github://mitsuhiko/jinja2') >>> import jinja2 >>> jinja2.__file__ 'github://mitsuhiko/jinja2/__init__.py' >>> from jinja2 import Template >>> t = Template('Hello from {{ hell }}!') >>> t.render(hell='Import Hook Hell') u'Hello from Import Hook Hell'

Monkeypatching

from a_horrible_library import SomeClass original_init = SomeClass.__init__

def new__init__(self, *args, **kwargs): original__init__(self, *args, **kwargs) self.something_else = Thing() SomeClass.__init__ = new__init__

__builtin__ patching

de = Translations.load(['de_DE', 'de', 'en']) import __builtin__ __builtin__._ = de.ugettext

import __builtin__ __builtin__.__import__ = my_fancy_import

Uncommon Magic

sys.modules behavior

import sys from os.path import join, dirname from types import ModuleType

class MagicModule(ModuleType): @property def git_hash(self): fn = join(dirname(__file__), '.git/refs/heads/master') with open(fn) as f: return f.read().strip()

old_mod = sys.modules[__name__] sys.modules[__name__] = mod = MagicModule(__name__) mod.__dict__.update(old_mod.__dict__)

>>> import magic_module >>> magic_module <module 'magic_module' from 'magic_module.py'> >>> magic_module.git_hash 'da39a3ee5e6b4b0d3255bfef95601890afd80709'

Custom Namespaces

from collections import MutableMapping

class CaseInsensitiveNamespace(MutableMapping): def __init__(self): self.ns = {} def __getitem__(self, key): return self.ns[key.lower()] def __delitem__(self, key): del self.ns[key.lower()] def __setitem__(self, key, value): self.ns[key.lower()] = value def __len__(self): return len(self.ns) def __iter__(self): return iter(self.ns)

exec ''' foo = 42 Bar = 23 print (Foo, BAR) ''' in {}, CaseInsensitiveNamespace()

Code Reloading

True Reloading
Design your application for reloading Acquire lock purge all entries in sys.modules with your module and that are linked to it. Reload the module in question and all dependencies.

Multiversioning

The Problem
Want multiple versions of the same library loaded. But libraries are actual Python modules and modules are cached in sys.modules So how can we do that?

The Plan
Import-hooks are no good. But __import__ can be monkey patched on __builtin__ And __import__ can use _getframe() or the globals dict to look into the callers namespace And can that way nd out what is the required version.

The Implementation

You really don't want to know

import multiversion multiversion.require('mylib', '2.0') # this will be version 2.0 of mylib import mylib

>>> import mylib >>> mylib.__name__ 'multiversion.space.mylib___322e30.mylib' >>> mylib.__file__ 'mylib-2.0/mylib.py'

Interpreter Warfare

Force Closures

def some_generated_code(): a = 1 b = 2 def inner_function(): if 0: unused(a, b) return locals() return inner_function

Patching Tracebacks

try: ... except Exception: exc_type, exc_value, tb = sys.exc_info() frames = make_frame_list(tb) frames = rewrite_frames(frames) prev_frame = None for frame in frames: if prev_frame is not None: tb_set_next(prev_frame, frame) prev_frame = frame

import sys import ctypes from types import TracebackType if hasattr(ctypes.pythonapi, 'Py_InitModule4_64'): _Py_ssize_t = ctypes.c_int64 else: _Py_ssize_t = ctypes.c_int class _PyObject(ctypes.Structure): pass _PyObject._fields_ = [ ('ob_refcnt', _Py_ssize_t), ('ob_type', ctypes.POINTER(_PyObject)) ] if hasattr(sys, 'getobjects'): class _PyObject(ctypes.Structure): pass _PyObject._fields_ = [ ('_ob_next', ctypes.POINTER(_PyObject)), ('_ob_prev', ctypes.POINTER(_PyObject)), ('ob_refcnt', _Py_ssize_t), ('ob_type', ctypes.POINTER(_PyObject)) ]

class _Traceback(_PyObject): pass _Traceback._fields_ = [ ('tb_next', ctypes.POINTER(_Traceback)), ('tb_frame', ctypes.POINTER(_PyObject)), ('tb_lasti', ctypes.c_int), ('tb_lineno', ctypes.c_int) ] def tb_set_next(tb, next): if not (isinstance(tb, TracebackType) and (next is None or isinstance(next, TracebackType))): raise TypeError('tb_set_next arguments must be tracebacks') obj = _Traceback.from_address(id(tb)) if tb.tb_next is not None: old = _Traceback.from_address(id(tb.tb_next)) old.ob_refcnt -= 1 if next is None: obj.tb_next = ctypes.POINTER(_Traceback)() else: next = _Traceback.from_address(id(next)) next.ob_refcnt += 1 obj.tb_next = ctypes.pointer(next)

Names of Variables

import gc, sys def find_names(obj): frame = sys._getframe(1) while frame is not None: frame.f_locals frame = frame.f_back result = set() for referrer in gc.get_referrers(obj): if isinstance(referrer, dict): for k, v in referrer.iteritems(): if v is obj: result.add(k) return tuple(result)

>>> a = [] >>> b = a >>> find_names(a) ('a', 'b')

Bytecode Hacks

from opcode import HAVE_ARGUMENT

def disassemble(code): code = map(ord, code) i = 0 n = len(code) while i < n: op = code[i] i += 1 if op >= HAVE_ARGUMENT: oparg = code[i] | code[i + 1] << 8 i += 2 else: oparg = None yield op, oparg

Implicit Self

class User(ImplicitSelf): def __init__(username, password): self.username = username self.set_password(password) def set_password(pw): self.hash = sha1(pw).hexdigest() def check_password(pw): return sha1(pw).hexdigest() == self.hash

def inject_self(code): varnames = ('self',) + tuple(n for i, n in enumerate(code.co_varnames)) names = tuple(n for i, n in enumerate(code.co_names)) bytecode = [] for op, arg in disassemble(code.co_code): if op in (LOAD_FAST, STORE_FAST): arg = varnames.index(code.co_varnames[arg]) elif op in (LOAD_GLOBAL, STORE_GLOBAL): if code.co_names[arg] == 'self': op = LOAD_FAST if op == LOAD_GLOBAL else STORE_FAST arg = 0 else: arg = names.index(code.co_names[arg]) elif op in (LOAD_ATTR, STORE_ATTR): arg = names.index(code.co_names[arg]) bytecode.append(chr(op)) if op >= opcode.HAVE_ARGUMENT: bytecode.append(chr(arg & 0xff)) bytecode.append(chr(arg >> 8)) return ''.join(bytecode), varnames, names

from types import CodeType, FunctionType

def implicit_self(function): code = function.func_code bytecode, varnames, names = inject_self(code) function.func_code = CodeType(code.co_argcount + 1, code.co_nlocals + 1, code.co_stacksize, code.co_flags, bytecode, code.co_consts, names, varnames, code.co_filename, code.co_name, code.co_firstlineno, code.co_lnotab, code.co_freevars, code.co_cellvars)

class ImplicitSelfType(type): def __new__(cls, name, bases, d): for key, value in d.iteritems(): if isinstance(value, FunctionType): implicit_self(value) return type.__new__(cls, name, bases, d)

class ImplicitSelf(object): __metaclass__ = ImplicitSelfType

Return Value Used?

def menu_items(): items = MenuItem.query.all() html = render_menu_items(items=items) if return_value_used(): return html print html

import sys, dis

def return_value_used(): frame = sys._getframe(2) code = frame.f_code.co_code[frame.f_lasti:] try: has_arg = ord(code[0]) >= dis.HAVE_ARGUMENT next_code = code[3 if has_arg else 1] except IndexError: return True return ord(next_code) != dis.opmap['POP_TOP']

What'll be my name?

>>> class Module(object): ... def __init__(self): ... self.name = assigned_name() ... >>> admin = Module() >>> admin.name 'admin'

import sys, dis

def assigned_name(): frame = sys._getframe(2) code = frame.f_code.co_code[frame.f_lasti:] try: has_arg = ord(code[0]) >= dis.HAVE_ARGUMENT skip = 3 if has_arg else 1 next_code = ord(code[skip]) name_index = ord(code[skip + 1]) except IndexError: return True if next_code in (dis.opmap['STORE_FAST'], dis.opmap['STORE_GLOBAL'], dis.opmap['STORE_NAME'], dis.opmap['STORE_DEREF']): namelist = frame.f_code.co_names if next_code == dis.opmap['STORE_GLOBAL']: namelist = frame.f_code.co_names elif next_code == dis.opmap['STORE_DEREF']: namelist = frame.f_code.co_freevars return namelist[name_index]

?
Questions :-)

Legal

Slides: http://lucumr.pocoo.org/talks/ Code: http://github.com/mitsuhiko/badideas armin.ronacher@active-4.com // @mitsuhiko