PyQueryableList: LINQ's Queryable List for Python
Software Engineering
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Introduction
PyQueryableList is a quick attempt to duplicate some of Microsoft's LINQ IQueryable natively in Python.
The main class is QueryableList which extends UserList, and accepts data in the constructor.
QueryableList makes use of a decorator returns_new to enable method chaining.
Code
__author__ = "Edward J. Stembler "
__copyright__ = "Copyright (c) 2009, Edward J. Stembler"
__license__ = "MIT"
__date__ = "$Apr 6, 2009 11:48 PM$"
__module_name__ = "Queryable List"
__version__ = "1.0"
from itertools import ifilter
from itertools import imap
from itertools import islice
import UserList
def returns_new(func):
def inner(self, args):
return QueryableList(list(func(self, args)))
return inner
class QueryableList(UserList.UserList):
def __init__(self, data):
super(QueryableList, self).__init__()
self.data = data
@returns_new
def take(self, count):
return islice(self.data, count)
@returns_new
def skip(self, count):
return islice(self.data, count, None)
@returns_new
def join(self, stream):
return ((left, right) for left in self.data for right in stream)
@returns_new
def select(self, func):
return imap(func, self.data)
@returns_new
def where(self, func):
return ifilter(func, self.data)
def single_or_default(self, func=None):
try:
return self.data[0] if func is None else self.where(func).data[0]
except IndexError:
return None
def single(self, func=None):
result = self.single_or_default(func)
if result is None or result == []:
raise ValueError
return result
def order_by(self, func):
self.data.sort(func)
return QueryableList(self.data)
def order_by_descending(self):
return self.order_by(lambda x, y: y - x)
def contains(self, item):
return self.where(lambda x: x == item) != []
def count(self, func=None):
return len(self.data) if func is None else self.where(func).count()
def sum(self, func=None):
return sum(self.data) if func is None else self.where(func).sum()
def average(self, func=None):
return self.sum() / len(self.data) if func is None else self.where(func).average() Usage
a = QueryableList(range(20)) # [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
b = a.take(10).where(lambda x: x < 5).order_by_descending() # Method chaining example
print b # [4, 3, 2, 1, 0]
print b.count() # 5
print b.count(lambda x: x < 2) # 2
print b.contains(4) # True
print b.contains(5) # False
print b.single() # 4
print b.single(lambda x: x < 3) # 2
print b.single_or_default(lambda x: x == 9) # None
try:
print b.single(lambda x: x == 9) # ValueError
except ValueError:
print "ValueError"
print b.sum() # 10
print b.sum(lambda x: x > 2) # 7
print b.average() # 2