Using comprehensions is often a way both to make code more compact and to shift our focus from the "how" to the "what". It is an expression that uses the same keywords as loop and conditional blocks, but inverts their order to focus on the data rather than on the procedure.
Simply changing the form of expression can often make a surprisingly large difference in how we reason about code and how easy it is to understand. The ternary operator also performs a similar restructuring of our focus, using the same keywords in a different order.
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# Original
doubled_numbers = []
for n in range(1,6):
doubled_numbers.append(n*2)
print(doubled_numbers)
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#list compreshensions
doubled_numbers = [n * 2 for n in range(1,12,2)] # 1 ,3, 5, 7, 9, 11
print(doubled_numbers)
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doubled_odds = []
for n in range(1,12):
if n % 2 == 1:
doubled_odds.append(n * 2)
print(doubled_odds)
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doubled_odds = [n * 2 for n in range(1,12) if n% 2 == 1]
print(doubled_odds)
!!!! Tip !!!!
: (line 4): (line 5)
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# FROM
numbers = range(2,10)
doubled_odds = []
for n in numbers:
if n % 2 == 1:
doubled_odds.append(n * 2)
print(doubled_odds)
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# TO
numbers = range(2,10)
doubled_odds = [n * 2 for n in numbers if n % 2 == 1]
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l = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 0]
lst = []
for v in l:
if v == 0 :
lst.append ('Zero')
else:
if v % 2 == 0:
lst.append ('even')
else:
lst.append ('odd')
print(lst)
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lst = ["zero" if v == 0 else "even" if v%2 == 0 else "odd" for v in l]
print(lst)
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print(['yes' if v == 1 else 'no' if v == 2 else 'idle' for v in l])
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def flatten_list_new(lst, result=None):
"""Flattens a nested list
>>> flatten_list([ [1, 2, [3, 4] ], [5, 6], 7])
[1, 2, 3, 4, 5, 6, 7]
"""
if result is None:
result = []
# else:
# result = [x if not isinstance(x, list) else flatten_list_new(x, list) for x in lst]
# result = [ x if not isinstance(x, list) else isinstance(x, list) for x in lst ]
result = [x for x in lst if not isinstance(x, list) else isinstance(x, list)]
# for x in lst:
# if isinstance(x, list):
# flatten_list_new(x, result)
# else:
# result.append(x)
return result
lst = [ [1, 2, [3, 4] ], [5, 6], 7]
print(flatten_list_new(lst))
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lst = []
for a in range(10):
if a % 2==0:
for x in range(a, 10):
lst.append(x)
print(lst)
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n = 10
lsts = [x for a in range(n) if a % 2 == 0 for x in range(a, 10)]
print(lsts)
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%%time
import os
files = []
for d in os.walk(r"E:\code\mj\lep\Section 1 - Core Python"):
for f in d[2]:
if f.endswith(".py"):
files.append(os.path.join(d[0], f))
print(len(files))
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%%time
import os
files = [os.path.join(d[0], f)
for d in os.walk(r"E:\code\mj\lep\Section 1 - Core Python")
for f in d[2] if f.endswith(".py")]
print(len(files))
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%%time
import os
files = [os.path.join(d[0], f)
for d in os.walk(r"E:\code\mj\lep\Section 1 - Core Python")
for f in d[2] if f.endswith(".py")]
print(len(files))
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%%time
restFiles = []
for d in os.walk(r"C:\apps"):
if "etc" in d[0]:
for f in d[2]:
if f.endswith(".txt"):
restFiles.append(os.path.join(d[0], f))
print(len(restFiles))
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%%time
restFiles = [os.path.join(d[0], f)
for d in os.walk(r"C:\apps")
if "etc" in d[0]
for f in d[2]
if f.endswith(".txt")]
print(len(restFiles))
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%%time
matrix = []
for row_idx in range(0, 3):
itmList = []
for item_idx in range(0, 3):
if item_idx == row_idx:
itmList.append(1)
else:
itmList.append(0)
matrix.append(itmList)
print(matrix)
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matrix = [ [ 1 if item_idx == row_idx
else 0 for item_idx in range(0, 3)]
for row_idx in range(0, 3) ]
print(matrix)
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lst = [1,2,34,4,5]
print(lst)
lst.append(2)
print(lst)
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lst.append(2)
print(lst)
l = set(lst)
print(l)
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names = [ 'aaLok', 'Manish', 'AalOK', 'Manish', 'Gupta', 'Johri', 'Mayank' ]
new_names1 = [name[0].upper() + name[1:].lower() for name in names if len(name) > 1 ]
new_names = sorted({name[0].upper() + name[1:].lower() for name in names if len(name) > 1 })
print(new_names1)
print(new_names)
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original = {'a':10, 'b': 34, 'A': 7, 'Z':3, "z": 199}
mcase_frequency = { k.lower() : original.get(k.lower(), 0) + original.get(k.upper(), 0) for k in original.keys() }
print(mcase_frequency)
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original = {'a':10, 'b': 34, 'A': 7, 'Z':3, "z": 199, 'c': 10}
flipped = {value: key for key, value in original.items()}
print(flipped)
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original = {'a':10, 'b': 34, 'A': 7, 'Z':3, "z": 199, 'c': 10}
newdict = {}
for key, value in original.items():
# print(ori)
if (value not in newdict):
newdict[value] = key
print(newdict)
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newdict = {value: key for key, value in original.items() if (value not in newdict)}
print(newdict)
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x = {"a": 10, "b": 20, "c": 20}
print(x)
x["a"] = 100
print(x)
This map doesn’t take a named function. It takes an anonymous, inlined function defined with lambda. The parameters of the lambda are defined to the left of the colon. The function body is defined to the right of the colon. The result of running the function body is (implicitly) returned.
The unfunctional code below takes a list of real names and appends them with randomly assigned code names.
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import random
names_dict = {}
names = ["Mayank", "Manish", "Aalok", "Roshan Musheer"]
code_names = ['Mr. Normal', 'Mr. God', 'Mr. Cool', 'The Big Boss']
random.shuffle(code_names)
for i in range(len(names)):
names_dict[names[i]] = code_names[i]
print(names_dict)
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import random
names_dict = {}
names = ["Mayank", "Manish", "Aalok", "Roshan Musheer"]
code_names = ['Mr. Normal', 'Mr. God', 'Mr. Cool', 'The Big Boss']
random.shuffle(code_names)
names_dict = dict(zip(names, code_names))
print(names_dict)
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ld = [{'a': 10, 'b': 20}, {'p': 10, 'u': 100}]
dict([kv for d in ld for kv in d.items()])
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x = (x**2 for x in range(20))
print(x)
print(list(x))
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itm = 10
print(itm / 2)
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def condition_based_on(itm):
return itm % 2 == 0
old_things = range(2,20, 3)
new_things = []
for ITEM in old_things:
if condition_based_on(ITEM):
new_things.append(ITEM)
print(new_things)
Can be rewritten into a list comprehension like this:
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new_things = [ITEM for ITEM in old_things if condition_based_on(ITEM)]
print(new_things)
NOTE
If you can nudge a for loop until it looks like the ones above, you can rewrite it as a list comprehension.