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item_24.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
from time import sleep
from datetime import datetime
def log(message, when=datetime.now()):
print(f'{when}: {message}')
log('Hi there!')
sleep(0.1)
log('Hello again!')
# >>>
# 2019-07-06 14:06:15.120124: Hi there!
# 2019-07-06 14:06:15.120124: Hello again!
# Example 2
def log(message, when=None):
# Document actual behavior in the docstring.
"""Log a message with a timestamp.
Args:
message: Message to print.
when: datetime of when the message occurred.
Defaults to the present time.
"""
if when is None:
when = datetime.now() # Allocate accordingly
print(f'{when}: {message}')
# Example 3
log('Hi there!')
sleep(0.1)
log('Hello again!')
# Example 4
import json
def decode(data, default={}):
try:
return json.loads(data)
except ValueError:
return default
# Example 5
foo = decode('bad data')
foo['stuff'] = 5
bar = decode('also bad')
bar['meep'] = 1
print('Foo:', foo)
print('Bar:', bar)
# >>>
# # foo & bar are equal to the default parameter: they are the same dictionary object (demonstrated below)
# Foo: {'stuff': 5, 'meep': 1}
# Bar: {'stuff': 5, 'meep': 1}
# Example 6
assert foo is bar
# Example 7: Correct approach
def decode(data, default=None):
"""Load JSON data from a string.
Args:
data: JSON data to decode.
default: Value to return if decoding fails.
Defaults to an empty dictionary.
"""
try:
return json.loads(data)
except ValueError:
if default is None:
default = {}
return default
# Example 8
foo = decode('bad data')
foo['stuff'] = 5
bar = decode('also bad')
bar['meep'] = 1
print('Foo:', foo)
print('Bar:', bar)
assert foo is not bar
# >>>
# Foo: {'stuff': 5}
# Bar: {'meep': 1}
# Example 9
# None default also works with type annotations
from typing import Optional
def log_typed(message: str,
when: Optional[datetime]=None) -> None: # only 2 valid choices for when are None or a datetime object
"""Log a message with a timestamp.
Args:
message: Message to print.
when: datetime of when the message occurred.
Defaults to the present time.
"""
if when is None:
when = datetime.now()
print(f'{when}: {message}')