Proper if..else syntax in Python

Question

In my python code, I am attempting to implement an if..else statement. See below:

 if index[y] == index[x]:
 continue
 else index[y] != index[x]
 indexdn = indexd
 indadd= index[y]-index[x]
 indexdn[:,y]=indexdn[:,y]+ indadd
 index[y,:]=-indexdn[:,y]
 indexdn[y,y]=0

However, no matter how many different ways I attempt to write else index[x] is not equal to index[y] I get a syntax error on return of this line. I have tried using else, elif, and for the not operand, != and 'is not'. What is the proper way to write a Python statement using a "not equal" operand so that I do not receive a syntax error?

• Stewart: Indentation is important in Python, unlike most other languages. The if and else need to be indented at the same level to be valid.
• TigerhawkT3: Have you tried the tutorial? Your indexing also looks funky. if..else troubles aside, does that part work?

Answer 1

The correct syntax is

if condition:
 # stuff
elif other:
 # stuff
elif some_other:
 # stuff
else:
 # stuff

Note that else does not get any explicit condition, it is the catch-all if none of the above conditions were True.

• heltonbiker: You could add to your answer the difference of using elif and else: if.
• Cory Kramer: @heltonbiker There is a good writeup of that here so I will defer to that
• heltonbiker: Yeah, my comment was just to remind the few cases where you actually want nested ifs instead of a single, switch/case-like set of conditions, since they are conceptually different.

Answer 2

Just fix your indentation, change else to elif and add the missing colon:

if index[y] == index[x]:
 continue
elif index[y] != index[x]: # indentation and colon
 indexdn = indexd
 indadd = index[y] - index[x]
 indexdn[:,y] = indexdn[:,y]+ indadd
 index[y,:] = -indexdn[:,y]
 indexdn[y,y] = 0

• heltonbiker: Also, since both tests are mutually exclusive, the OP could do straigth with an else, or even with no additional test at all.
• Zizouz212: @heltonbiker And if there are more clauses? What if there's an extra condition. Then unwanted execution will take place.
• Stefan Pochmann: @heltonbiker In most cases they should be mutually exclusive, but they might not be. See this, for example.

Answer 3

I think you are missing upon 2 important things 1. Intendation and 2. Colon after else

if index[y] == index[x]:
 continue
else:
 indexdn = indexd
 indadd= index[y]-index[x]
 indexdn[:,y]=indexdn[:,y]+ indadd
 index[y,:]=-indexdn[:,y]
 indexdn[y,y]=0

By going by the example given by you - no comparison is required in the else.

• Pralhad Narsinh Sonar: @Brandon Ginley - please let me know if there is any further scope that you are looking after to implement if...else in python.

Answer 4

I guess you use this in a loop, but if you dont you should use the keyword pass instead continue.

if index[y] == index[x]:
 pass
elif: # indentation and colon
 indexdn = indexd
 indadd = index[y] - index[x]
 indexdn[:, y] = indexdn[:, y] + indadd
 index[y, :] = -indexdn[:, y]
 indexdn[y, y] = 0

However, if it isn't in loop, you need only:

if index[y] != index[x]:
 indexdn = indexd
 indadd = index[y] - index[x]
 indexdn[:, y] = indexdn[:, y] + indadd
 index[y, :] = -indexdn[:, y]
 indexdn[y, y] = 0