关于re.finditer只有一个参数的疑问

Question

我正在学习流畅的python这本书，书里有串代码

import sys
import re

WORD_RE = re.compile(r'w+')

index = {}
with open(sys.argv[1],encoding = 'utf-8') as fp:

for line_no,line in enumerate(fp,1):
    **for match in WORD_RE.finditer(line):**
        word = match.group()
        

我加粗的这段代码，用到的finditer（）方法只传入了一个参数，得到的是什么，pattern是有默认值吗？

Answer 1

WORD_RE = re.compile(r'w+')
WORD_RE.finditer(line)

你用的是compile后pattern对象的finditer方法
上面两行相当于re.finditer(r'w+', line)
pattern没有默认值，返回的是一个match对象迭代器。

Answer 2

看一下官方文档：

re.finditer(pattern, string, flags=0)
Return an iterator yielding match objects over all non-overlapping matches for the RE pattern in string. The string is scanned left-to-right, and matches are returned in the order found. Empty matches are included in the result.

Changed in version 3.7: Non-empty matches can now start just after a previous empty match.

再看一下re.findall：

re.findall(pattern, string, flags=0)
Return all non-overlapping matches of pattern in string, as a list of strings. The string is scanned left-to-right, and matches are returned in the order found. If one or more groups are present in the pattern, return a list of groups; this will be a list of tuples if the pattern has more than one group. Empty matches are included in the result.

Changed in version 3.7: Non-empty matches can now start just after a previous empty match.

所以，这两者的功能是一样的，都是返回某个字符串中的全部匹配结果。不同之处在于re.findall返回一个列表，而re.finditer返回一个迭代器。

WORD_RE.finditer实际上是Pattern.finditer方法，WORD_RE是一个Pattern对象（re.compile返回一个Pattern对象），然后在这个对象上调用finditer方法。Pattern对象的方法和re模块的方法几乎是一一对应的。Pattern.finditer方法和re.finditer方法的唯一区别就是由于pattern已知，无需再传递pattern，只需要一个位置参数string就可以了。

Answer 3

finditer返回迭代器，那个参数好像是默认的。