python最大递归深度错误“maximum recursion depth exceeded while calling a Python object”

使用str方法强制转换一个BeautifulSoup对象成字符串的时候报错了，提示是“maximum recursion depth exceeded while calling a Python object”，意思大致是“当调用该对象超过最大递归深度”

报错如下所示：

  Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "C:\Python27\lib\site-packages\bs4\element.py", line 1045, in __str__
    return self.encode()
  File "C:\Python27\lib\site-packages\bs4\element.py", line 1055, in encode
    u = self.decode(indent_level, encoding, formatter)
  File "C:\Python27\lib\site-packages\bs4\element.py", line 1126, in decode
    indent_contents, eventual_encoding, formatter)
  File "C:\Python27\lib\site-packages\bs4\element.py", line 1195, in decode_contents
    formatter))
  File "C:\Python27\lib\site-packages\bs4\element.py", line 1126, in decode
  ......
  
  File "C:\Python27\lib\site-packages\bs4\element.py", line 1126, in decode
    indent_contents, eventual_encoding, formatter)
  File "C:\Python27\lib\site-packages\bs4\element.py", line 1195, in decode_contents
    formatter))
  File "C:\Python27\lib\site-packages\bs4\element.py", line 1098, in decode
    text = self.format_string(val, formatter)
  File "C:\Python27\lib\site-packages\bs4\element.py", line 163, in format_string
    output = formatter(s)
  File "C:\Python27\lib\site-packages\bs4\element.py", line 120, in substitute_xml
    ns, EntitySubstitution.substitute_xml)
  File "C:\Python27\lib\site-packages\bs4\element.py", line 104, in _substitute_if_appropriate
    if (isinstance(ns, NavigableString)
RuntimeError: maximum recursion depth exceeded while calling a Python object

其实原因是：在Python里的递归调用是有限制的，可以使用sys模块里的getrecursionlimit方法查看，代码如下所示：

1	sys.getrecursionlimit()

打开终端运行Python，可以看到默认限制值为：1000

Python 3.8.3 (tags/v3.8.3:6f8c832, May 13 2020, 22:37:02) [MSC v.1924 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.getrecursionlimit()
1000
>>>