On Wed, Oct 07, 2020 at 01:59:24PM +1100, Chris Angelico wrote:
> On Wed, Oct 7, 2020 at 1:46 PM Steven D'Aprano <st...@pearwood.info> wrote:
> > > Recall that we already have literal expressions such as
> > > >>> [1, 2, 3] # List literal
> > > >>> (1, 2, 3) # Tuple literal
> > > >>> {1, 2, 3} # Set literal
> >
> > Point of terminology: these are not literals (although informally people
> > often call them that, including me on occassion). They are *displays*.
[...]
> > The constants 1, 2 and 3 are literals and the compiler can insert them
> > directly into the code. The list [1, 2, 3] is not, the compiler has to
> > insert code to create the list at runtime.
> >
>
> Be careful with this, as it's only a one-way proof. Any literal can
> and will be inserted directly as a constant, but there are some
> non-literals that can be as well:
Sure, I believe that is a function of the keyhole optimizer: it can
recognise certain non-literals, such as complex numbers and certain sets
and tuples, and pre-generate them at compile time. That's a nice little
optimization, especially swapping a mutable set for a frozen set because
the compiler sees that the set is only ever read from, never mutated.
(I think that was one of Serhiy's optimizations? Whoever it was, thank
you.)
> Complex numbers and frozen sets don't technically have literals, but
> thanks to compiler magic, they can appear to. (Also, raw string
> literals are a thing, but in the compiled code, what you get is just a
> string object, further confusing this type of proof.)
Raw strings are just an alternative syntax for strings. Just as we can
write the same int as `0x4e` or `78`, or the same float as `12e-2` or
`0.12`, or the same string as `"""abc"""` or `'\x61\x62\x63'`. They're
not different "things", just different ways of writing the same thing.
> Although when it comes to f-strings, I honestly don't know what counts
> as a "literal". Is a single f-string a literal?
An f-string is executable code.
py> string = f"π/2 = {print('Calculating...') or
__import__('math').atan(float('INF'))}"
Calculating...
py> string
'π/2 = 1.5707963267948966'
Think of it as somewhat analogous to a list comprehension, in that it is
an expression that contains executable code that runs at runtime, but it
returns a string instead of a list, and rather than a for loop, you can
include an arbitrary number of executable expressions.
So not much like a list comprehension but a lot like calling eval on
string literals :-)
> But hey, it wouldn't be python-ideas without pedantry :)
Technically, there are lots of other mailing lists that are equally
pedantic *wink*
--
Steve
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