Re: Linq to Python
- From: Duncan Booth <duncan.booth@xxxxxxxxxxxxxxx>
- Date: 24 Sep 2008 20:59:12 GMT
r0g <aioe.org@xxxxxxxxxxxxxxxxxx> wrote:
OK so maybe I'm being naive here but it looks to me like this new
paradigm's big idea is to use a python + SQL type syntax to access data
in random objects. Big whoop. It's not that difficult to write a
generators that wraps XML files and databases is it?
What am I missing here?
Simple LINQ expressions like the one you gave map easily to Python list
comprehensions. What Microsoft have done though is provide a consistent
implementation which allows you to write complex SQL like expressions which
will work identically on databases or most other sequence types.
Here's another LINQ example:
List<Customer> customers = GetCustomerList();
var customerOrderGroups =
from c in customers
select
new {c.CompanyName,
YearGroups =
from o in c.Orders
group o by o.OrderDate.Year into yg
select
new {Year = yg.Key,
MonthGroups =
from o in yg
group o by o.OrderDate.Month into mg
select new { Month = mg.Key, Orders = mg }
}
};
ObjectDumper.Write(customerOrderGroups, 3);
I'm not overly keen on SQL syntax: I think this sort of complex expression
is hard to read. LINQ has a pretty straightforward conversion to method
calls, and for me this consistent set of methods is the important thing
rather than the SQL syntax. e.g. 'group by' maps directly to a call to a
method GroupBy(), so another of Microsoft's LINQ examples is:
public void Linq45() {
string[] anagrams = {"from ", " salt", " earn ", " last ",
" near ", " form "};
var orderGroups = anagrams.GroupBy(
w => w.Trim(),
a => a.ToUpper(),
new AnagramEqualityComparer()
);
ObjectDumper.Write(orderGroups, 1);
}
public class AnagramEqualityComparer : IEqualityComparer<string>
{
public bool Equals(string x, string y) {
return getCanonicalString(x) == getCanonicalString(y);
}
public int GetHashCode(string obj) {
return getCanonicalString(obj).GetHashCode();
}
private string getCanonicalString(string word) {
char[] wordChars = word.ToCharArray();
Array.Sort<char>(wordChars);
return new string(wordChars);
}
}
Python still wins hands down on this example both in verbosity and
readability:
" near ", " form "]anagrams = ["from ", " salt", " earn ", " last ",
return sorted(w.strip().upper())from itertools import groupby
def AnagramKey(w):
key=AnagramKey):for k,words in groupby(sorted(anagrams, key=AnagramKey),
for w in words:
print w.strip().upper()
print "..."
EARN
NEAR
....
SALT
LAST
....
FROM
FORM
....
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.
.
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