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item_87.py
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executable file
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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
# my_module.py
def determine_weight(volume, density):
if density <= 0:
raise ValueError('Density must be positive')
try:
determine_weight(1, 0)
except ValueError:
pass
else:
assert False
# Example 2
# my_module.py
class Error(Exception):
"""Base-class for all exceptions raised by this module."""
class InvalidDensityError(Error):
"""There was a problem with a provided density value."""
class InvalidVolumeError(Error):
"""There was a problem with the provided weight value."""
def determine_weight(volume, density):
if density < 0:
raise InvalidDensityError('Density must be positive')
if volume < 0:
raise InvalidVolumeError('Volume must be positive')
if volume == 0:
density / volume
# Example 3
class my_module:
Error = Error
InvalidDensityError = InvalidDensityError
@staticmethod
def determine_weight(volume, density):
if density < 0:
raise InvalidDensityError('Density must be positive')
if volume < 0:
raise InvalidVolumeError('Volume must be positive')
if volume == 0:
density / volume
try:
weight = my_module.determine_weight(1, -1)
except my_module.Error:
logging.exception('Unexpected error')
else:
assert False
# Example 4
SENTINEL = object()
weight = SENTINEL
try:
weight = my_module.determine_weight(-1, 1)
except my_module.InvalidDensityError:
weight = 0
except my_module.Error:
logging.exception('Bug in the calling code')
else:
assert False
assert weight is SENTINEL
# Example 5
try:
weight = SENTINEL
try:
weight = my_module.determine_weight(0, 1)
except my_module.InvalidDensityError:
weight = 0
except my_module.Error:
logging.exception('Bug in the calling code')
except Exception:
logging.exception('Bug in the API code!')
raise # Re-raise exception to the caller
else:
assert False
assert weight == 0
except:
logging.exception('Expected')
else:
assert False
# Example 6
# my_module.py
class NegativeDensityError(InvalidDensityError):
"""A provided density value was negative."""
def determine_weight(volume, density):
if density < 0:
raise NegativeDensityError('Density must be positive')
# Example 7
try:
my_module.NegativeDensityError = NegativeDensityError
my_module.determine_weight = determine_weight
try:
weight = my_module.determine_weight(1, -1)
except my_module.NegativeDensityError:
raise ValueError('Must supply non-negative density')
except my_module.InvalidDensityError:
weight = 0
except my_module.Error:
logging.exception('Bug in the calling code')
except Exception:
logging.exception('Bug in the API code!')
raise
else:
assert False
except:
logging.exception('Expected')
else:
assert False
# Example 8
# my_module.py
class Error(Exception):
"""Base-class for all exceptions raised by this module."""
class WeightError(Error):
"""Base-class for weight calculation errors."""
class VolumeError(Error):
"""Base-class for volume calculation errors."""
class DensityError(Error):
"""Base-class for density calculation errors."""