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item_13.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
try:
car_ages = [0, 9, 4, 8, 7, 20, 19, 1, 6, 15]
car_ages_descending = sorted(car_ages, reverse=True)
oldest, second_oldest = car_ages_descending #error cannot unpack rest of list to var need *var
except:
logging.exception("Expected")
else:
assert False
print(second_oldest)
# use *second_oldest to get excess list of items
car_ages = [0, 9, 4, 8, 7, 20, 19, 1, 6, 15]
car_ages_descending = sorted(car_ages, reverse=True)
oldest, *second_oldest = car_ages_descending #assign list to vars
print(oldest, second_oldest)
# Example 2
# split items over variables last var is list
oldest = car_ages_descending[0] #every part of list is individually assigned below
second_oldest = car_ages_descending[1]
others = car_ages_descending[2:]
print(oldest, second_oldest, others)
# Example 3
# use * for list assignent
oldest, second_oldest, *others = car_ages_descending #now assign different parts of list in one line
print(oldest, second_oldest, others)
# Example 4
# first,others[],last
oldest, *others, youngest = car_ages_descending #can breakup assignment of list
print(oldest, youngest, others)
# others[],forlast,last
*others, second_youngest, youngest = car_ages_descending #berakup list by choice
print(youngest, second_youngest, others)
# Example 5
try:
# This will not compile no division of items so use no *
source = """*others = car_ages_descending""" #there is just one list assigned to one var no *var
eval(source)
except:
logging.exception("Expected")
else:
assert False
# Example 6
try:
# This will not compile # source iunclear in divided items
source = """first, *middle, *second_middle, last = [1, 2, 3, 4]""" #cannot use *var twice not clear what to assign to *vars
eval(source)
except:
logging.exception("Expected")
else:
assert False
# Example 7
car_inventory = {
"Downtown": ("Silver Shadow", "Pinto", "DMC"),
"Airport": ("Skyline", "Viper", "Gremlin", "Nova"),
}
# unpack items from dict
((loc1, (best1, *rest1)), (loc2, (best2, *rest2))) = car_inventory.items() #multiple var* in key/tuple combinations one *var per tuple
print(f"Best at {loc1} is {best1}, {len(rest1)} others")
print(f"Best at {loc2} is {best2}, {len(rest2)} others")
# Example 8
# spli list over variables even if no rest []
short_list = [1, 2]
first, second, *rest = short_list #if list is short rest can be assigned to *var even if *var is []
print(first, second, rest)
# Example 9
# it as only 2 items more items gives error too many items
it = iter(range(1, 3))
[i for i in range(1, 4)]
#first, second = it
first, second,*other = it #will have [] for other
print(it)
print(f"{first} and {second} and {other}")
# Example 10
def generate_csv(): #create csv with heading and 100 lines per yield
yield ("Date", "Make", "Model", "Year", "Price")
for i in range(100):
yield ("2019-03-25", "Honda", "Fit", "2010", "$3400")
yield ("2019-03-26", "Ford", "F150", "2008", "$2400")
# Example 11
# instanciate the function then print else only object id
all_csv_rows = list(generate_csv())
print(generate_csv) #prints object
print(all_csv_rows)#header and 200 lines
# first row/item
header = all_csv_rows[0] #first item of list
# rest of the rows/items
rows = all_csv_rows[1:] #rest of list
print("CSV Header:", header)
print("Row count: ", len(rows))
# Example 12
it = generate_csv()
# unpack the items in one go by * assign
header, *rows = it #assign direct header and rest to vars in one line
print("CSV Header:", header)
print("Row count: ", len(rows))