6 ms·

I am but an egg, but isn't this the same but shorter? def f(L=None): L = L or []

coderdude 14y ago
It might work the same depending on your use of it but it's not the same. In that instance L will become a blank list if it is equal to None, zero, or a zero length string. There are many cases where this wouldn't affect anything, but there can also be instances where that will cause you to define L as a blank list when you really wanted to keep L's value. I think it's always better to be explicit and test for the value(s) you expect.
- chernevik 14y ago
  Ah. These are the little assumptions that keep blowing off my feet. Thanks.
  - SoftwareMaven 14y ago
    It is these cases that brought the ternary operator to Python: def f(x=None): x if x is not None else []
    - iki23 14y ago
      Yes, but why? Actually, it's No, because python culture aims to use one way to do thing, the least surprising one. In this case it's: def f(x=None): if x is None: x = []
      - SoftwareMaven 14y ago
        Because that "if" is a statement whereas the ternary expression is, well, an expression. There are places expressions can be used that statements can't (eg lambdas) and that x or [] won't work (eg when x is False). That said, people still seem to favor your form as the more Pythonic way. Personally, I think that's just because the ternary expression is relatively new.
    - jongraehl 14y ago
      you forgot 'return'
      - SoftwareMaven 14y ago
        Yeah, I guess my typing went into "lambda mode" since it was a one liner.
    - chernevik 14y ago
      I'm a little confused about this as an assignment to a variable. Is that because of the 'return' omission referred to below? If all function f did was return x, I could see how this works with a 'return' prepended. But if this expression is a line in a larger function, and is intended to reset the value of argument x if no other value is passed in for it, does this really act as an assigment to x? Because I sort of read this expression as evaluating to some value -- the passed value for x, or a [] -- but does this assign that value to the argument x? Or must it be x = [expression]
      - coderdude 14y ago
        def f(x=None): x = x if x is not None else [] return x Now it will assign that value back to x. Otherwise it would just evaluate the expression.
        chernevik 14y ago
        Thanks for the additional clarity.
jsprinkles 14y ago
I originally had "don't do that!" in my comment with your exact code, and edited it out for brevity because I've only seen a couple people do it (and they understood the ramifications, which others have told you). If you're interested in brevity, this is as terse as it gets: L = [] if L is None else L

It might work the same depending on your use of it but it's not the same. In that instance L will become a blank list if it is equal to None, zero, or a zero length string. There are many cases where this wouldn't affect anything, but there can also be instances where that will cause you to define L as a blank list when you really wanted to keep L's value. I think it's always better to be explicit and test for the value(s) you expect.

chernevik 14y ago
Ah. These are the little assumptions that keep blowing off my feet. Thanks.
- SoftwareMaven 14y ago
  It is these cases that brought the ternary operator to Python: def f(x=None): x if x is not None else []
  - iki23 14y ago
    Yes, but why? Actually, it's No, because python culture aims to use one way to do thing, the least surprising one. In this case it's: def f(x=None): if x is None: x = []
    - SoftwareMaven 14y ago
      Because that "if" is a statement whereas the ternary expression is, well, an expression. There are places expressions can be used that statements can't (eg lambdas) and that x or [] won't work (eg when x is False). That said, people still seem to favor your form as the more Pythonic way. Personally, I think that's just because the ternary expression is relatively new.
  - jongraehl 14y ago
    you forgot 'return'
    - SoftwareMaven 14y ago
      Yeah, I guess my typing went into "lambda mode" since it was a one liner.
  - chernevik 14y ago
    I'm a little confused about this as an assignment to a variable. Is that because of the 'return' omission referred to below? If all function f did was return x, I could see how this works with a 'return' prepended. But if this expression is a line in a larger function, and is intended to reset the value of argument x if no other value is passed in for it, does this really act as an assigment to x? Because I sort of read this expression as evaluating to some value -- the passed value for x, or a [] -- but does this assign that value to the argument x? Or must it be x = [expression]
    - coderdude 14y ago
      def f(x=None): x = x if x is not None else [] return x Now it will assign that value back to x. Otherwise it would just evaluate the expression.
      - chernevik 14y ago
        Thanks for the additional clarity.

jsprinkles 14y ago

I originally had "don't do that!" in my comment with your exact code, and edited it out for brevity because I've only seen a couple people do it (and they understood the ramifications, which others have told you). If you're interested in brevity, this is as terse as it gets: L = [] if L is None else L