Python Comprehensions
Internal
TODO
- PROCESS PyOOP "Comprehensions" + "List comprehensions" + "Set and dictionary comprehensions"
Overview
A comprehension is a compact way of creating a data structure from one or more iterators. They are essentially loops with a more compact syntax.
List Comprehensions
A list comprehension produces a list from an iterable type by applying an expression to each of the elements, and optionally a condition. The iterable instance must be indeed iterable, otherwise we get an error message, shown below. So None does not work, it produces a runtime error.
TypeError: 'NoneType' object is not iterable
Simple List Comprehension
[<expression> for <var> in <iterable>]
The list comprehension moves the loop inside square brackets.
This example iterates over a list of custom object instances and produces a comma-separated list of their __str__() representations, which printing the list with print() does not do:
class C:
def __init__(self, i):
self.i = i
def __str__(self):
return str(self.i)
l = [C(1), C(2), C(3)]
print(l)
displays:
[<__main__.C object at 0x102f06b20>, <__main__.C object at 0x1030159d0>, <__main__.C object at 0x10302c9a0>]
The following list comprehension:
print(', '.join([str(i) for i in l]))
displays:
1, 2, 3
Conditional List Comprehension
[<expression> for <var> in <iterable> if <condition>]
An example of a conditional list comprehension is to generate a list of even numbers from a list of numbers:
l = list(range(10))
print(l)
displays:
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
To get the even numbers:
l2 = [i for i in l if i % 2 == 0]
print(l2)
displays:
[0, 2, 4, 6, 8]
Nested List Comprehensions
Just as there can be nested loops, there can be more than on set of for ... clauses.
rows = range(1, 4)
cols = range(1, 3)
cells = [(row, col) for row in rows for col in cols]
Use Comprehensions for Side Effects
The purpose of a comprehension is to generate a list from another list. However, it can be also used to exercise side effects on the elements. The result list is discarded.
class Box:
def __init__(self):
self.color = None
def paint(self, value):
self.color = value
boxes = [Box(), Box(), Box()]
[b.paint('blue') for b in boxes]
for b in boxes:
assert b.color == 'blue'
Dictionary Comprehensions
{<key-expression>: <value-expression> for <var> in <iterable>}
{<key-expression>: <value-expression> for <var> in <iterable> if <condition>}
- PROCESS IPy Comprehensions Page 87.
A dictionary comprehension starts from a list, and for each element of the list, generates a key and a value, which are then inserted in the dictionary. The key and the value are introduced with the (k, v) syntax:
l = ['a', 'b', 'c']
d = dict([(k, k.upper()) for k in l])
print(d)
will print:
{'a': 'A', 'b': 'B', 'c': 'C'}
Set Comprehensions
{<expression> for <var> in <iterable>}
{<expression> for <var> in <iterable>} if <condition>
- PROCESS IPy Comprehensions Page 87.