Linq to Python

[Duncan Booth]

"Tim Rowe" <digitig at gmail.com> wrote:

>> I haven't yet had occasion to use LINQ in anger yet, so I have no
>> idea whether its an idea to love or to hate. I do think it is good
>> that C# has effectively sprouted list comprehensions (not to mention
>> anonymous types and type inferencing) and I expect there may be some
>> aspects worth looking at for Python but I think they are more likely
>> to lead to itertools functions than extensions to syntax.
> 
> Yes, looking at what LINQ adds to C# (according to
> http://msdn.microsoft.com/en-gb/library/bb397909.aspx):
> - Implicitly typed variables: Python already has.
> - Object and collection initialisers: Not sure whether Python can do
> this directly, but it can certainly emulate it with a dictionary.
> - Anonymous types: Not sure whether Python can do this directly, but
> it can certainly emulate it with a dictionary.
> - Extension methods: Python already has.
> - Lambda expressions: Python already has.
> - Auto-Implemented properties: No, but that's just syntactic sugar to
> make declarations more compact.
> 
> So all of the language elements that are needed for LINQ are already
> in Python; a library should do the trick.
> 
Not quite. The C# implementation of lambda expressions has a neat trick 
that is central to LINQ. A lambda expression compiles either to executable 
code, or to an expression tree. If you are filtering some C# sequence 
object then (as with Python lambdas) the LINQ code simply calls the 
compiled lambda expression like any other delegate, but if you are 
filtering a SQL lookup the LINQ code gets an expression syntax tree and 
further compiles it (at runtime) into SQL code to perform the filtering at 
the database level.

Python can't do the compile time matching to vary what it produces 
according to how the lambda is used, but it would be perfectly possible for 
function objects which are compiled from a lambda expression to grow an 
extra attribute that would hold an AST for the expression. That way you 
could write code that took a filter function and, if it had the AST 
attribute available, compiled it to SQL or xpath or whatever else you 
fancied.