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item_26.py
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#!/usr/bin/env PYTHONHASHSEED=1234 python3
# Copyright 2014-2019 Brett Slatkin, Pearson Education Inc.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# Reproduce book environment
import random
random.seed(1234)
import logging
from pprint import pprint
from sys import stdout as STDOUT
# Write all output to a temporary directory
import atexit
import gc
import io
import os
import tempfile
TEST_DIR = tempfile.TemporaryDirectory()
atexit.register(TEST_DIR.cleanup)
# Make sure Windows processes exit cleanly
OLD_CWD = os.getcwd()
atexit.register(lambda: os.chdir(OLD_CWD))
os.chdir(TEST_DIR.name)
def close_open_files():
everything = gc.get_objects()
for obj in everything:
if isinstance(obj, io.IOBase):
obj.close()
atexit.register(close_open_files)
# Example 1
def trace(func):
def wrapper(*args, **kwargs):
result = func(*args, **kwargs)
print(f'{func.__name__}({args!r}, {kwargs!r}) '
f'-> {result!r}')
return result
return wrapper
# Example 2
@trace
def fibonacci(n):
"""Return the n-th Fibonacci number"""
if n in (0, 1):
return n
return (fibonacci(n - 2) + fibonacci(n - 1))
# Example 3
def fibonacci(n):
"""Return the n-th Fibonacci number"""
if n in (0, 1):
return n
return fibonacci(n - 2) + fibonacci(n - 1)
fibonacci = trace(fibonacci) # trace returns the wrapper defined within its body
# wrapper gets assigned to the fibonacci name in the containing module because of the decorator
# this is problematic because it undermines tools that do introspection, such as debuggers
# Example 4
fibonacci(4)
# Example 5
print(fibonacci)
# >>>
# <function trace.<locals>.wrapper at 0x108955dc0>
# Example 6: Unexpected behavior
help(fibonacci) # should return fibonacci's docstring ("Return the n-th Fibonacci number") but doesn't
# >>>
# Help on function wrapper in module __main__: # runs help on wrapper instead of fibonacci
# # because fibonacci is decorated bytrace
#
# wrapper(*args, **kwargs)
# Example 7
try:
import pickle
pickle.dumps(fibonacci)
except:
logging.exception('Expected')
else:
assert False
# Example 8: Correct approach yielding expected behavior
from functools import wraps
def trace(func):
@wraps(func)
def wrapper(*args, **kwargs):
result = func(*args, **kwargs)
print(f'{func.__name__}({args!r}, {kwargs!r}) '
f'-> {result!r}')
return result
return wrapper
@trace # fibonacci is decorated
def fibonacci(n):
"""Return the n-th Fibonacci number"""
if n in (0, 1):
return n
return fibonacci(n - 2) + fibonacci(n - 1)
# Example 9
help(fibonacci)
# >>>
# Help on function fibonacci in module __main__:
# fibonacci(n)
# Return the n-th Fibonacci number
# Example 10
print(pickle.dumps(fibonacci)) # pickle is an object serializer
# >>>
# b'\x80\x04\x95\x1a\x00\x00\x00\x00\x00\x00\x00\x8c\x08__main__\
# ➥x94\x8c\tfibonacci\x94\x93\x94.