Triple-double quote v.s. Double quote

They're both strings, so there is no difference. The preferred style is triple double quotes (PEP 257):

For consistency, always use """triple double quotes""" around docstrings.

Use r"""raw triple double quotes""" if you use any backslashes in your docstrings. For Unicode docstrings, use u"""Unicode triple-quoted strings""".

From the PEP8 Style Guide:

• PEP 257 describes good docstring conventions. Note that most importantly, the """ that ends a multiline docstring should be on a line by itself, e.g.:

  """Return a foobang
  
  Optional plotz says to frobnicate the bizbaz first.
  """
  

• For one liner docstrings, it's okay to keep the closing """ on the same line.

PEP 257 recommends using triple quotes, even for one-line docstrings:

• Triple quotes are used even though the string fits on one line. This makes it easy to later expand it.

Note that not even the Python standard library itself follows these recommendations consistently. For example,

• abcoll.py
• ftplib.py
• functools.py
• inspect.py