将断言用于控制流 [英] Use assert for control flow

问题描述

我正在尝试检查单词是否带有尾部分号。我在长字符串中找到单词的位置，我想检查单词单词开头+单词长度处的字符是否为：。可能是单词是字符串中的最后一个单词，因此尝试获取下一个字符将引发 IndexException

I am trying to check if a word has a trailing semicolon. I have the position of the word within a long string, and I want to check if the character at the position start of word + length of word is a :. It can be that the word is the last word in the string, and therefore trying to get the next char will raise an IndexException

我有三个想法：

1）检查我们不在字符串末尾，然后检查它是否不是分号

semicolon_pos = pos_word + len(word) # Possible position
if semicolon_pos < len(long_string) and long_string[semicolon_pos] == ':':
    # do something...
else:
    # do something else

不是通常所说的pythonic

Not what it is normally considered pythonic

2）试试-除分号分号的提取和等式

semicolon_pos = pos_word + len(word) # Possible position
try:
   if long_string[semicolon_pos] == ':':
      # Do something
   else:
      # do something else (which is the same as in the except clause)

except IndexError:
    # do something else

有点有点奇怪，提取然后嵌套。而且我必须做一些技巧来避免代码重复

Looks a little bit weird, to extract and then nested. And I have to do a little trick to avoid the code duplication

3）尝试除分号提取和断言以确保它是分号

semicolon_pos = pos_word + len(word) # Possible position
try:
    assert long_string[semicolon_pos] == ':'
    # do something
except (IndexError, AssertionError):
    # do something else

结构更平整，很容易阅读，但是我有滥用特例的感觉。

Structure is flatter and it is easily read, but I have the feeling that is abusing exceptions usage.

该怎么办蟒蛇人说？
预先表示感谢。

What do the python folks say? thanks in advance.

推荐答案

最简单的方法是将切片用作下标。切片不会抛出IndexError：

The simplest way is just to use a slice as a subscript. Slices don't throw IndexError:

semicolon_pos = pos_word + len(word) # Possible position
if long_string[semicolon_pos:semicolon_pos+1] == ':':
   # Do something
else:
   # do something else 

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