14.07.19 07:06, Andrew Barnert via Python-ideas пише:
(Re-sending, because this was originally a reply to an off-list message
by Nima Hamidi)
On Jul 13, 2019, at 14:12, Nima Hamidi
<ham...@stanford.edu
<mailto:ham...@stanford.edu>> wrote:
Sometimes it's necessary not to evaluate the expression. Two such
applications of NSE in R are as follows:
1. Data-tables have cleaner syntax. For example, letting dt be a
data-table with a column called price, one can retrieve items cheaper
than $1 using the following: dt [price < 1]. Pandas syntax requires
something like dt[dt.price < 1]. This is currently inevitable as the
expression is evaluated *before* __getitem__ is invoked. Using NSE,
dt.__getitem__ can, first, add its columns to locals() dictionary and
then evaluate the expression in the new context.
This one looks good. I can also imagine it being useful for SQLAlchemy,
appscript, etc. just as it is for Pandas.
But in your proposal, wouldn’t this have to be written as dt[`price <
1`]? I think the cost of putting the expression in ticks is at least as
bad as the cost of naming the dt.
Also: dt.price < 1 is a perfectly valid expression, with a useful value.
You can store it in a temporary variable to avoid repeating it, or stash
it for later, or print it out to see what’s happening. But price < 1 on
its own is a NameError, and I’m not sure what `price < 1` is worth on
its own. Would this invite code that’s hard to refactor and even harder
to debug?
The more interesting problem is that in general case you have not simple
`price < 1`, but `price < x` where x is a variable or more complex
expression. price should be evaluated at the callee context while x
should be evaluated at the caller context. And how Python can know what
is what?
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