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item_022.py
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#!/usr/bin/env PYTHONHASHSEED=1234 python3
# Copyright 2014-2024 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.
### Start book environment setup
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)
### End book environment setup
print("Example 1")
try:
search_key = "red"
my_dict = {"red": 1, "blue": 2, "green": 3}
for key in my_dict:
if key == "blue":
my_dict["yellow"] = 4 # Causes error
except:
logging.exception('Expected')
else:
assert False
print("Example 2")
try:
my_dict = {"red": 1, "blue": 2, "green": 3}
for key in my_dict:
if key == "blue":
del my_dict["green"] # Causes error
except:
logging.exception('Expected')
else:
assert False
print("Example 3")
my_dict = {"red": 1, "blue": 2, "green": 3}
for key in my_dict:
if key == "blue":
my_dict["green"] = 4 # Okay
print(my_dict)
print("Example 4")
try:
my_set = {"red", "blue", "green"}
for color in my_set:
if color == "blue":
my_set.add("yellow") # Causes error
except:
logging.exception('Expected')
else:
assert False
print("Example 5")
my_set = {"red", "blue", "green"}
for color in my_set:
if color == "blue":
my_set.add("green") # Okay
print(my_set)
print("Example 6")
my_list = [1, 2, 3]
for number in my_list:
print(number)
if number == 2:
my_list[0] = -1 # Okay
print(my_list)
print("Example 7")
my_list = [1, 2, 3]
bad_count = 0
for number in my_list:
print(number)
if number == 2:
my_list.insert(0, 4) # Causes error
# Break out of the infinite loop
bad_count += 1
if bad_count > 5:
print("...")
break
print("Example 8")
my_list = [1, 2, 3]
for number in my_list:
print(number)
if number == 2:
my_list.append(4) # Okay this time
print(my_list)
print("Example 9")
my_dict = {"red": 1, "blue": 2, "green": 3}
keys_copy = list(my_dict.keys()) # Copy
for key in keys_copy: # Iterate over copy
if key == "blue":
my_dict["green"] = 4 # Modify original dict
print(my_dict)
print("Example 10")
my_list = [1, 2, 3]
list_copy = list(my_list) # Copy
for number in list_copy: # Iterate over copy
print(number)
if number == 2:
my_list.insert(0, 4) # Inserts in original list
print(my_list)
print("Example 11")
my_set = {"red", "blue", "green"}
set_copy = set(my_set) # Copy
for color in set_copy: # Iterate over copy
if color == "blue":
my_set.add("yellow") # Add to original set
print(my_set)
print("Example 12")
my_dict = {"red": 1, "blue": 2, "green": 3}
modifications = {}
for key in my_dict:
if key == "blue":
modifications["green"] = 4 # Add to staging
my_dict.update(modifications) # Merge modifications
print(my_dict)
print("Example 13")
my_dict = {"red": 1, "blue": 2, "green": 3}
modifications = {}
for key in my_dict:
if key == "blue":
modifications["green"] = 4
value = my_dict[key]
if value == 4: # This condition is never true
modifications["yellow"] = 5
my_dict.update(modifications) # Merge modifications
print(my_dict)
print("Example 14")
my_dict = {"red": 1, "blue": 2, "green": 3}
modifications = {}
for key in my_dict:
if key == "blue":
modifications["green"] = 4
value = my_dict[key]
other_value = modifications.get(key) # Check cache
if value == 4 or other_value == 4:
modifications["yellow"] = 5
my_dict.update(modifications) # Merge modifications
print(my_dict)