variables in an if statement

Question

Is it bad practise to assign variables in an if statement and not use each variable if that part of the if statement is called?

For example in the following code I have two code option. Option 1 if trigger == ‘1’ or if trigger == ‘2’ then variable y will be unused as it is only needed if trigger == ‘4’ or if trigger == ‘5’. Option 2 is longer, but fixes this.

# option 1

if trigger == '1' or trigger == '4':
    x = int(start)
    y = int(s1)
elif trigger == '2'or trigger == '5':
    x = int(s1)
    y = int(s2)
elif trigger == '3':
    x = int(s2)

# option 2

if trigger == '1' or trigger == '4':
    x = int(start)
elif trigger == '2'or trigger == '5':
    x = int(s1)
elif trigger == '3':
    x = int(s2)

if trigger == '4':
    y = int(s1)
elif trigger == '5':
    y = int(s2)

My question is which one is correct and which one should I use?

Thanks.

Update –

So is it ok to set a variable and not use it if that part of the if function (in this case) is called? It is not bad practice or lead to memory issues etc. Thanks.

Looks fine to me. I would do if trigger in ('1', '4'):, however, it's more pythonic. — Joel Cornett
– Joel Cornett, Commented Oct 11, 2012 at 9:39
@JoelCornett: if trigger in '14' does the same thing and requires fewer keystrokes — inspectorG4dget
– inspectorG4dget, Commented Oct 11, 2012 at 9:42
@inspectorG4dget - but that fails in the case that trigger == "14". — detly
– detly, Commented Oct 11, 2012 at 9:46
@detly: trigger=='14' is not a requirement in this problem; and if were, I would suggest if trigger in '1 4 14'.split(). It still takes fewer keystrokes and makes it easier to add more trigger values to check against. — inspectorG4dget
– inspectorG4dget, Commented Oct 11, 2012 at 9:49
Optimizing for keystrokes is hardly in the spirit of Python, or this question. — tripleee
– tripleee, Commented Oct 11, 2012 at 9:53

Joachim Isaksson · Accepted Answer · 2012-10-11 09:42:52Z

2

If you really want to have them defined only if used, why not make it totally explicit and obvious both to read and change?

if   trigger == '1':
    x = int(start)

elif trigger == '2':
    x = int(s1)

elif trigger == '3':
    x = int(s2)

elif trigger == '4':
    x = int(start)
    y = int(s1)

elif trigger == '5':
    x = int(s1)
    y = int(s2)

answered Oct 11, 2012 at 9:42

Joachim Isaksson

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1 Comment

Tom Fin Over a year ago

Well I don’t necessarily need them to be defined only if used. ATM I am trying to reduce my already existing code and am now wondering if there are any side effects or if it is bad programming practice to define a variable and not use them every time a function is called. I have updated by original post to reflect this.

Rohit Jain · Accepted Answer · 2012-10-11 09:44:15Z

1

Why not have one more condition inside your if.

This is a better way to write your option 2. Which I think you can follow: -

if trigger in ('1', '4'): // You can better use `in` to check for more values
    x = int(start)

    if trigger == '4': 
        y = int(s1)

elif trigger in ('2', '5'):  
    x = int(s1)

    if trigger == '5': 
        y = int(s2)            

elif trigger == '3':
    x = int(s2)

edited Oct 11, 2012 at 9:44

answered Oct 11, 2012 at 9:38

Rohit Jain

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Comments

glglgl · Accepted Answer · 2012-10-11 09:45:19Z

1

In analogy to the optimizations in the KV diagram where Xed fields are considered or not depending if it is useful in optimizations (cf. 7-segment display with BCD values where the values 10..15 are undefined), I would tend to keep it as simple as possible and assign values even if they are not used later.

answered Oct 11, 2012 at 9:45

glglgl

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2 Comments

Tom Fin Over a year ago

@ glglgl So there are no issus with assign values even if they are not used later?

glglgl Over a year ago

No, they are just left unused.

Ben · Accepted Answer · 2012-10-12 08:55:43Z

0

If all of s1, s2 etc will always exist and be valid integers, then option 1 is fine.

However, it does make your code depend unnecessarily on the format of values you don't use. Sometimes that's fine. But if e.g. s1 and friends come from options passed on the command line or entered in an input file, your users would not like you for throwing a ValueError for leaving those values undefined, forcing them to pass meaningless integers in parameters they have no use for.

So really, the answer is that it all depends on the context! It's hard to give a general answer to these kinds of question.

answered Oct 12, 2012 at 8:55

Ben

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1 Comment

Tom Fin Over a year ago

Thanks your answer has cleared up reasons for and against using it.

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