文檔字符串。注意，是?__doc__?，前后各兩個下劃線。

一般而言，是對函數/方法/模塊所實現功能的簡單描述。但當指向具體對象時，會顯示此對象從屬的類型的的文檔字符串。(示例見以下?a.__doc__)

>>>?str.__doc__
"str(string[,?encoding[,?errors]])?->?str\n\nCreate?a?new?string?object?from?the?given?encoded?string.\nencoding?defaults?to?the?current?default?string?encoding.\nerrors?can?be?'strict',?'replace'?or?'ignore'?and?defaults?to?'strict'."

>>>?import?math
>>>?math.__doc__
'This?module?is?always?available.??It?provides?access?to?the\nmathematical?functions?defined?by?the?C?standard.'

>>>?a?=?[1]

>>>?a.count.__doc__
'L.count(value)?->?integer?--?return?number?of?occurrences?of?value'
>>>?a.__doc__
"list()?->?new?empty?list\nlist(iterable)?->?new?list?initialized?from?iterable's?items"

為自定義的函數創建?__doc__?的方法示例：

>>>?def?func():
????"""Here's?a?doc?string"""
????pass
>>>?func.__doc__
"Here's?a?doc?string"