Syntax of one line IF condition in Python

Question

Can someone say why this doesn't work in Python? Is it simply invalid syntax or is there more to it?

arr[0] += 12 if am_or_pm == 'PM'

The error message:

 File "solution.py", line 13
 arr[0] += 12 if am_or_pm == 'PM'
 ^
SyntaxError: invalid syntax 

This works:

if am_or_pm == 'PM': arr[0] += 12

• Ted Klein Bergman: That's not valid syntax. Maybe you're thinking of ternary conditional operation? arr[0] += 12 if am_or_pm == 'PM' else 0
• Nicolas Heimann: for better readability you may use if in a separate line, but the ternary conditional operator Ted suggested is fine as well.
• M3RS: Thanks guys, that helps. So I either need to specify an 'else' branch to make it a ternary operation or go with the classic if syntax. Thanks again!
• phipsgabler: @Andras Exactly. The reason is that the conditional expression is an expression. Leaving out the else would result in the "expression" having no defined value; e.g., what should print('hi' if False) sensibly do (if we refrain from using None everywhere automatically, which is a design choice)?

Answer 1

There is surely a kind of usage in Python that the if and else clause is in the same line. This is used when you need to assign a value to a variable under certain conditions. Like this

a = 1 if b == 1 else 2

This means if b is 1, a will be 1, else, a will be 2.
But if and else must all be written to form the valid syntax.

Answer 2

python doesn't have semicolon. So there is break line into the same line. python consider code into a same line that written in a line. so here a expression and a conditional statement in a same line that is not valid. python interpreter doesn't recognize what the code exactly mean. you can use separate line for this.

arr[0] += 12 
if am_or_pm == 'PM':