4 ms·

I sometimes use your "bug" on purpose in tests, when checking that two values are either both defined or both undefined, but not one defined and one undefined:

I sometimes use your "bug" on purpose in tests, when checking that two values are either both defined or both undefined, but not one defined and one undefined: $ python >>> a, b = 1, 2 >>> bool(a) ^ bool(b) False >>> a, b = 1, 1 >>> bool(a) ^ bool(b) False >>> a, b = None, None >>> bool(a) ^ bool(b) False >>> a, b = None, 1 >>> bool(a) ^ bool(b) True (Note: this doesn't work if a value can be 0, because bool(0) is False in Python). This helps to avoid having to write something like `(a is not None and b is not None) or (a is None and b is None)`.

shiftingleft 6y ago
Your note is very important, I think it would be good to give it more emphasis. What works flawlessly, however, is: In [1]: a = None In [2]: b = 0 In [3]: (a is None) ^ (b is None) Out[3]: True Alternatively, as suggested in another comment, you can use inequality as a replacement for XOR: In [4]: (a is None) != (b is None) Out[4]: True Another error prone pattern is the following: In [5]: a or b Out[5]: 0 Which can behave differently depending on the order of elements/values: In [6]: b or a Out[6]: None # should be 0 Since b is zero, it doesn't count. Less error prone, but also more verbose is: In [10]: a if a is not None else b Out[10]: 0
- sleavey 6y ago
  Lesson for myself: check my working. The test I actually used the XOR in, that I was referring to, uses the first example you wrote - I agree, this is much better. However, I also like your second example, which is arguably clearer. Thanks!
gfodor 6y ago
Not a python person, but I usually do this in other languages via an equality check `(a != nil) == (b != nil)` etc
patrec 6y ago
You are aware that ^ on a 1 bit value (like a boolean) is just !=, right?
- sleavey 6y ago
  Hah, I wrote that code on the fly and didn't check the aforementioned test implementation, where I had `(a is None) ^ (b is None)` like the other commenter suggested.
  - patrec 6y ago
    You could just write this as (a is None) is not (b is None) or (a is None) != (b is None) It won't make a difference (and is 1 to to 5 characters longer), but it seems "cleaner" since it's more specific.

shiftingleft 6y ago

Your note is very important, I think it would be good to give it more emphasis. What works flawlessly, however, is: In [1]: a = None In [2]: b = 0 In [3]: (a is None) ^ (b is None) Out[3]: True Alternatively, as suggested in another comment, you can use inequality as a replacement for XOR: In [4]: (a is None) != (b is None) Out[4]: True Another error prone pattern is the following: In [5]: a or b Out[5]: 0 Which can behave differently depending on the order of elements/values: In [6]: b or a Out[6]: None # should be 0 Since b is zero, it doesn't count. Less error prone, but also more verbose is: In [10]: a if a is not None else b Out[10]: 0

sleavey 6y ago
Lesson for myself: check my working. The test I actually used the XOR in, that I was referring to, uses the first example you wrote - I agree, this is much better. However, I also like your second example, which is arguably clearer. Thanks!

gfodor 6y ago

Not a python person, but I usually do this in other languages via an equality check `(a != nil) == (b != nil)` etc

patrec 6y ago

You are aware that ^ on a 1 bit value (like a boolean) is just !=, right?

sleavey 6y ago
Hah, I wrote that code on the fly and didn't check the aforementioned test implementation, where I had `(a is None) ^ (b is None)` like the other commenter suggested.
- patrec 6y ago
  You could just write this as (a is None) is not (b is None) or (a is None) != (b is None) It won't make a difference (and is 1 to to 5 characters longer), but it seems "cleaner" since it's more specific.