style, formatting the slice operator

Question

PEP 8 doesn't mention the slice operator. From my understanding, unlike other operators, it should not be surrounded with whitespace

spam[3:5]   # OK
spam[3 : 5] # NOT OK

Does this hold when using complex expressions, that is, which one is considered better style

     1. spam[ham(66)//3:44+eggs()]
     2. spam[ham(66) // 3: 44 + eggs()]
     3. spam[ham(66) // 3 : 44 + eggs()]
     4. something else?

Answer 1

As you already mentioned, PEP8 doesn't explicitly mention the slice operator in that format, but spam[3:5] is definitely more common and IMHO more readable.

If pep8 checker is anything to go by, the space before : will be flagged up

[me@home]$ pep8  <(echo "spam[3:44]")   # no warnings
[me@home]$ pep8  <(echo "spam[3 : 44]")  
/dev/fd/63:1:7: E203 whitespace before ':'

... but that's only because of it assumes : to be the operator for defining a literal dict and no space is expected before the operator. spam[3: 44] passes for that reason, but that just doesn't seem right.

On that count, I'd stick to spam[3:44].

---

Nested arithmetic operations are a little trickier. Of your 3 examples, only the 2nd one passes PEP8 validation:

[me@home]$ pep8 <(echo "spam[ham(66)//3:44+eggs()]")
/dev/fd/63:1:13: E225 missing whitespace around operator

[me@home]$ pep8 <(echo "spam[ham(66) // 3:44 + eggs()]")  # OK

[me@home]$ pep8 <(echo "spam[ham(66) // 3 : 44 + eggs()]")
/dev/fd/63:1:18: E203 whitespace before ':'

However, I find all of the above difficult to parse by eye at first glance.

For readability and compliance with PEP8, I'd personally go for:

 spam[(ham(66) // 3):(44 + eggs())]

Or for more complication operations:

 s_from = ham(66) // 3 
 s_to = 44 + eggs()
 spam[s_from:s_to]