Using "and" and "or" operator with Python strings [duplicate]

Question

I don't understand the meaning of the line:

parameter and (" " + parameter) or "" 

where parameter is string

Why would one want to use and and or operator, in general, with python strings?

Answer 1

Suppose you are using the value of parameter, but if the value is say None, then you would rather like to have an empty string "" instead of None. What would you do in general?

if parameter:     # use parameter (well your expression using `" " + parameter` in this case else:     # use "" 

This is what that expression is doing. First you should understand what and and or operator does:

• a and b returns b if a is True, else returns a.
• a or b returns a if a is True, else returns b.

So, your expression:

parameter and (" " + parameter) or "" 

which is effectively equivalent to:

(parameter and (" " + parameter)) or  "" #    A1               A2               B #           A                     or   B 

How the expression is evaluated if:

• parameter - A1 is evaluated to True:

  result = (True and " " + parameter) or ""  result = (" " + parameter) or ""  result = " " + parameter 

• parameter - A1 is None:

  result = (None and " " + parameter) or ""  result = None or ""  result = "" 

---

As a general suggestion, it's better and more readable to use A if C else B form expression for conditional expression. So, you should better use:

" " + parameter if parameter else "" 

instead of the given expression. See PEP 308 - Conditional Expression for motivation behind the if-else expression.