How to write inline if statement for print?

Question

I need to print some stuff only when a boolean variable is set to True. So, after looking at this, I tried with a simple example:

>>> a = 100 >>> b = True >>> print a if b   File "<stdin>", line 1     print a if b              ^ SyntaxError: invalid syntax   

Same thing if I write print a if b==True.

What am I missing here?

Answer 1

Python does not have a trailing if statement.

There are two kinds of if in Python:

1. if statement:

   if condition: statement if condition:     block 

2. if expression (introduced in Python 2.5)

   expression_if_true if condition else expression_if_false 

And note, that both print a and b = a are statements. Only the a part is an expression. So if you write

print a if b else 0 

it means

print (a if b else 0) 

and similarly when you write

x = a if b else 0 

it means

x = (a if b else 0) 

Now what would it print/assign if there was no else clause? The print/assignment is still there.

And note, that if you don't want it to be there, you can always write the regular if statement on a single line, though it's less readable and there is really no reason to avoid the two-line variant.

Answer 2

Inline if-else EXPRESSION must always contain else clause, e.g:

a = 1 if b else 0 

If you want to leave your 'a' variable value unchanged - assing old 'a' value (else is still required by syntax demands):

a = 1 if b else a 

This piece of code leaves a unchanged when b turns to be False.