some piece of my codes:

            if self.tagname and self.tagname2 in list1:
                try: 
                    question = soup.find("div", "post-text")
                    title = soup.find("a", "question-hyperlink")
                    self.list2.append(str(title)+str(question)+url)
                    current += 1
                except AttributeError:
                    pass            
            logging.info("%s questions passed, %s questions \
                collected" % (count, current))
            count += 1
        return self.list2

pep8 warning is:

trailing whitespace 37:try
trailing whitespace 43:pass

Can you please tell me what is this?

Trailing whitespace is any spaces or tabs after the last non-whitespace character on the line until the newline.

In your posted question, there is one extra space after try:, and there are 12 extra spaces after pass:

>>> post_text=""'\
...             if self.tagname and self.tagname2 in list1:
...                 try: 
...                     question = soup.find("div", "post-text")
...                     title = soup.find("a", "question-hyperlink")
...                     self.list2.append(str(title)+str(question)+url)
...                     current += 1
...                 except AttributeError:
...                     pass            
...             logging.info("%s questions passed, %s questions \
...                 collected" % (count, current))
...             count += 1
...         return self.list2
... '''
>>> for line in post_text.splitlines():
...     if line.rstrip() != line:
...         print(repr(line))
... 
'                try: '
'                    pass            '

See where the strings end? There are spaces before the lines (indentation), but also spaces after.

Use your editor to find the end of the line and backspace. Many modern text editors can also automatically remove trailing whitespace from the end of the line, for example every time you save a file.

Trailing whitespace:

It is extra spaces (and tabs) at the end of line      
                                                 ^^^^^ here

Strip them:

#!/usr/bin/env python2
"""\
strip trailing whitespace from file
usage: stripspace.py <file>
"""

import sys

if len(sys.argv[1:]) != 1:
  sys.exit(__doc__)

content=""
outsize = 0
inp = outp = sys.argv[1]
with open(inp, 'rb') as infile:
  content = infile.read()
with open(outp, 'wb') as output:
  for line in content.splitlines():
    newline = line.rstrip(" \t")
    outsize += len(newline) + 1
    output.write(newline + '\n')

print("Done. Stripped %s bytes." % (len(content)-outsize))

https://gist.github.com/techtonik/c86f0ea6a86ed3f38893

This is just a warning and it doesn’t make problem for your project to run, you can just ignore it and continue coding.
But if you’re obsessed about clean coding, same as me, you have two options:

  1. Hover the mouse on warning in VS Code or any IDE and use quick fix to remove white spaces.
  2. Press f1 then type trim trailing whitespace.

I have got similar pep8 warning W291 trailing whitespace

long_text=""'Lorem Ipsum is simply dummy text  <-remove whitespace
of the printing and typesetting industry.'''

Try to explore trailing whitespaces and remove them. ex: two whitespaces at the end of Lorem Ipsum is simply dummy text

White spaces do not affect your coding function, but removing them definitely makes your code much cleaner and avoids unwanted small mistakes.

If you use VS Code, you can put this into your settings.json file to automatically trim all the white spaces whenever you save a file.

"files.trimTrailingWhitespace": true