Effective Python 读书笔记 Item 17:Be Defensive When Iterating Over Arguments

#!/usr/bin/env python3 # Copyright 2014 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. # Preamble to mimick book environment import logging from pprint import pprint from sys import stdout as STDOUT # Example 1 def normalize(numbers): total = sum(numbers) result = [] for value in numbers: percent = 100 * value / total result.append(percent) return result # Example 2 visits = [15, 35, 80] percentages = normalize(visits) print(percentages) # Example 3 path = 'my_numbers.txt' with open(path, 'w') as f: for i in (15, 35, 80): f.write('%d\n' % i) def read_visits(data_path): with open(data_path) as f: for line in f: yield int(line) # Example 4 it = read_visits('my_numbers.txt') percentages = normalize(it) print(percentages) # Example 5 it = read_visits('my_numbers.txt') print(list(it)) print(list(it)) # Already exhausted # Example 6 def normalize_copy(numbers): numbers = list(numbers) # Copy the iterator total = sum(numbers) result = [] for value in numbers: percent = 100 * value / total result.append(percent) return result # Example 7 it = read_visits('my_numbers.txt') percentages = normalize_copy(it) print(percentages) # Example 8 def normalize_func(get_iter): total = sum(get_iter()) # New iterator result = [] for value in get_iter(): # New iterator percent = 100 * value / total result.append(percent) return result # Example 9 percentages = normalize_func(lambda: read_visits(path)) print(percentages) # Example 10 class ReadVisits(object): def __init__(self, data_path): self.data_path = data_path def __iter__(self): with open(self.data_path) as f: for line in f: yield int(line) # Example 11 visits = ReadVisits(path) percentages = normalize(visits) print(percentages) # Example 12 def normalize_defensive(numbers): if iter(numbers) is iter(numbers): # An iterator -- bad! raise TypeError('Must supply a container') total = sum(numbers) result = [] for value in numbers: percent = 100 * value / total result.append(percent) return result # Example 13 visits = [15, 35, 80] normalize_defensive(visits) # No error visits = ReadVisits(path) normalize_defensive(visits) # No error # Example 14 try: it = iter(visits) normalize_defensive(it) except: logging.exception('Expected') else: assert False

def normalize(numbers): total = sum(numbers) result = [] for value in numbers: percent = 100 * value / total result.append(percent) return result