为什么这个python代码有效？

Question

# 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.

Answer 1

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 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 .

Answer 2

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 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

Answer 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)

Answer 4

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)