4 ms·

> assert l and len(l) == 1 This is redundant: if a list's length is 1, then it's true in a boolean context. Also, please stop naming your lists 'l'. On a vas

> assert l and len(l) == 1 This is redundant: if a list's length is 1, then it's true in a boolean context. Also, please stop naming your lists 'l'. On a vast array of fonts, it differs only in a few pixels from '1'. Use "L" instead :)

j_baker 16y ago
I wouldn't name a list l in real code. It was just the first thing that popped to mind. :-) And as was mentioned the "assert l" part is defending against l being None. I suppose I could be more explicit by saying "assert l is not None and len(l) == 1".
- jemfinch 16y ago
  Yes, you should definitely should be more explicit if that's what you intend to check: PEP8 makes that explicit. I'm not sure why you'd defend against None anyway. Why defend against None, but not against 3.1459 or 4j or ''?
  - statictype 16y ago
    There's nothing wrong with `is l`. Sounds like you're just splitting hairs. He's defending against None because calling __len__ on None results in an exception. 3.14159 also results in an exception but it's far more likely that the object passed was None than that it was a completely different type than the one expected.
    - jemfinch 16y ago
      > it's far more likely that the object passed was None than that it was a completely different type than the one expected. FYI, None is a completely different type than the one expected.
durin42 16y ago
He's defending against None with the assert l and len(l) bit, or at least that's what I'd assume.
- swolchok 16y ago
  If he's defending against None, he should assert l is not None and len(l) == 1
statictype 16y ago
This is redundant: if a list's length is 1, then it's true in a boolean context. In a boolean context, the list is true if the length is non-zero. This example and the one the article is about is for the case where you know the list to have exactly one element. Not zero and not more than one.
- jemfinch 16y ago
  You are aware that asserting `len(L) == 1` excludes the possibility that it's empty, right? You know what "redundant" means, right?
  - statictype 16y ago
    if L is None then asserting `len(L)==1` throws an exception. The `if L and` part safeguards against that.
    - jemfinch 16y ago
      And the point that was made in the other branch of this thread is that if the author intends to guard against None (who knows why?) then he should say, explicitly, "if L is not None". That's what PEP8 recommends precisely to avoid ambiguities such as these.
dirtyaura 16y ago
There's a better variable naming scheme for generic lists: Use xs. If you have multiple lists use ys etc. This has multiple benefits over L in terms of readability anad understandability as a single-item variable names can be made to match the list naming scheme: for x in xs: for y in ys: do_some_fancy_calculation(x,y)

j_baker 16y ago

I wouldn't name a list l in real code. It was just the first thing that popped to mind. :-) And as was mentioned the "assert l" part is defending against l being None. I suppose I could be more explicit by saying "assert l is not None and len(l) == 1".

jemfinch 16y ago
Yes, you should definitely should be more explicit if that's what you intend to check: PEP8 makes that explicit. I'm not sure why you'd defend against None anyway. Why defend against None, but not against 3.1459 or 4j or ''?
- statictype 16y ago
  There's nothing wrong with `is l`. Sounds like you're just splitting hairs. He's defending against None because calling __len__ on None results in an exception. 3.14159 also results in an exception but it's far more likely that the object passed was None than that it was a completely different type than the one expected.
  - jemfinch 16y ago
    > it's far more likely that the object passed was None than that it was a completely different type than the one expected. FYI, None is a completely different type than the one expected.

durin42 16y ago

He's defending against None with the assert l and len(l) bit, or at least that's what I'd assume.

swolchok 16y ago
If he's defending against None, he should assert l is not None and len(l) == 1

statictype 16y ago

This is redundant: if a list's length is 1, then it's true in a boolean context. In a boolean context, the list is true if the length is non-zero. This example and the one the article is about is for the case where you know the list to have exactly one element. Not zero and not more than one.

jemfinch 16y ago
You are aware that asserting `len(L) == 1` excludes the possibility that it's empty, right? You know what "redundant" means, right?
- statictype 16y ago
  if L is None then asserting `len(L)==1` throws an exception. The `if L and` part safeguards against that.
  - jemfinch 16y ago
    And the point that was made in the other branch of this thread is that if the author intends to guard against None (who knows why?) then he should say, explicitly, "if L is not None". That's what PEP8 recommends precisely to avoid ambiguities such as these.

dirtyaura 16y ago

There's a better variable naming scheme for generic lists: Use xs. If you have multiple lists use ys etc. This has multiple benefits over L in terms of readability anad understandability as a single-item variable names can be made to match the list naming scheme: for x in xs: for y in ys: do_some_fancy_calculation(x,y)