3 
# this code I don't understand 
def cls():                       #if statement freaks me out
    os.system('cls' if os.name=='nt' else 'clear')

I understand that it works but not WHY it works. After several hours of perusing google, python docs, and stackoverflow, I am none the wiser. I have been unable to find anything explaining that manner of using an IF statement in a function call like this. I have run it under 2.7 and 3.5 so it does not appear to be specific to a particular version. I have seen similar stuff done with for loops sort of get that it might be a standard way of doing things. I kind of like it myself. Just don't understand how it works.

I am confused about how the IF statement is evaluated within system call and I am especially confused about the syntax of the IF statement. I have not seen that syntax in any of the documentation I have read.

Python is completely new to me. So forgive me if this is bonehead simple. But I don't get it.

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2
  • os.system expects a command, the inline if generates one. Commented Dec 19, 2016 at 6:41
  • It's just an if statement. system expects only a string and the expression delivers it whichever way the if goes. Commented Dec 19, 2016 at 6:52

4 Answers 4

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4

If you are more familiar with C-like languages and their ternary operator, Python's a if b else c is similar to b ? a : c. Therefore, the code above says:

if we are on NT/Windows
  then use `cls`
else (for Linux, etc) use `clear`

The result is then passed to the os.system() command to perform the OS-specific operation.

Here is some tutorial information on this subject.

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2 Comments

Wow...y'all are fast!
Was not allowed to edit my first line comment. Hit enter too soon. The c analogy works for me. I knew what the code was doing but did not know this syntax and was not smart enough to find he docs. Thank you very much for both the answer and the link.
4 
A if C else B

This first evaluates C; if it is true, A is evaluated to give the result, otherwise, B is evaluated to give the result.

This shortcut conditional expression syntax was added since Python 2.5 Check it here

There's similar syntax in other languages, taking Java for example: min = (a<b)? a: b which checks whether a is smaller than b, it returns a if a is smaller, otherwise returns b. BTW, it is call ternary operator in java.

In your case:

'cls' if os.name=='nt' else 'clear' it checks whether os.name equals to string nt, if it is, it returns cls, otherwise it returns clear

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1 Comment

Thanks. The link was especially helpful. Ken Y-N used the C style operator to explain (b ? a : c) which I thought was a good analogy too.
3

The if expression is not executed inside of the system call. It is executed before the system call. Your code is equivalent to

command = 'cls' if os.name=='nt' else 'clear'
os.system(command)

The if expression itself is only a short form for the if statement

if os.name=='nt':
    command = 'cls'
else:
    command = 'clear'
os.system(command)
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3

this inline if - else is the python version of ternary operator.

in languages like C/C++/Java/JavaScript you would write

a = b > c ? 10 : 20

in python you would write the same as

a = 10 if b > c else 20

you can use the same construct to pass parameters to functions.

in C/C++

foo(b > c ? 10 : 20)

in python

foo(10 if b > c else 20)
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