4.14. Idiom Patterns¶

Python for loop is equivalent to forEach in other languages
Other languages for loop is Python's while (sic!)

For:

for (int i = 0; i <= 10; i++)  # C/C++/Java
for (var i = 0; i <= 10; i++)  # JavaScript

i = 0
while i <= 10:  #  Python
    i += 1

ForEach Index:

for (let idx in collection)         # JavaScript
for idx in range(len(collection))   # Python

ForEach Element:

for (var element : collection)   # Java
for (let element of collection)  # JavaScript
for element in collection        # Python
for i in range(0,10)             # Python

Enumerate:

for (int i = 0; i <= collection.length; i++)  # C++ / JAVA / JavaScript
    element = collection[i]

for (let i in collection)  # JavaScript
    element = collection[i]

for i in range(len(collection)):  # Python (1to1 algorithm conversion)
    element = collection[i]

for i, element in enumerate(collection)  # Python (Pythonic way)

Zip:

for (int i = 0; i <= collection.length; i++)  # C++ / JAVA / JavaScript
    a = collection1[i]
    b = collection2[i]

for i in range(len(collection1)):  # Python (1to1 algorithm conversion)
    a = collection1[i]
    b = collection2[i]

for a, b in zip(collection1, collection2)  # Python (Pythonic way)

Code Complexity vs. Programmer Experience:

../../_images/idiom-patterns-programmer-exp.png

Time Complexity: https://wiki.python.org/moin/TimeComplexity

4.14.1. Loop Patterns¶

>>> data = ['a', 'b', 'c']
>>> i = 0
>>> result = []
>>>
>>> while i < len(data):
...     x = data[i]
...     result.append(x.upper())
...     i += 1
>>>
>>> print(result)
['A', 'B', 'C']

>>> data = ['a', 'b', 'c']
>>> result = []
>>>
>>> for x in data:
...     result.append(x.upper())
>>>
>>> print(result)
['A', 'B', 'C']

>>> data = ['a', 'b', 'c']
>>> result = [x.upper() for x in data]
>>>
>>> print(result)
['A', 'B', 'C']

4.14.2. Range¶

>>> i = 0
>>>
>>> while i < 5:
...     i += 1

>>> for i in range(5):
...     pass

4.14.3. ForEach¶

>>> DATA = ['a', 'b', 'c']
>>>
>>> for i in range(len(DATA)):
...     value = DATA[i]

>>> DATA = ['a', 'b', 'c']
>>>
>>> for value in DATA:
...     pass

4.14.4. Sum¶

>>> DATA = [1, 2, 3]
>>> result = 0
>>>
>>> for i in range(len(DATA)):
...     result += DATA[i]

>>> DATA = [1, 2, 3]
>>> result = sum(DATA)

4.14.5. Enumerate¶

>>> DATA = ['a', 'b', 'c']
>>> i = 0
>>>
>>> while i < len(DATA):
...     value = DATA[i]
...     i += 1

>>> DATA = ['a', 'b', 'c']
>>>
>>> for i, value in enumerate(DATA):
...     pass

4.14.6. Zip¶

>>> header = ['a', 'b', 'c']
>>> values = [1, 2, 3]
>>> result = {}
>>>
>>> for i in range(len(header)):
...     key = header[i]
...     val = values[i]
...     result[key] = value

>>> header = ['a', 'b', 'c']
>>> values = [1, 2, 3]
>>>
>>> result = zip(header, values)
>>> dict(result)
{'a': 1, 'b': 2, 'c': 3}

4.14.7. List Comprehension¶

>>> DATA = ['a', 'b', 'c']
>>> result = []
>>>
>>> for x in DATA:
...     result.append(x)
...
>>> result
['a', 'b', 'c']

>>> DATA = ['a', 'b', 'c']
>>>
>>> result = [x for x in DATA]
>>> result
['a', 'b', 'c']

4.14.8. Set Comprehension¶

>>> DATA = ['a', 'b', 'c']
>>> result = set()
>>>
>>> for x in DATA:
...     result.add(x)

>>> DATA = ['a', 'b', 'c']
>>> result = {x for x in DATA}

4.14.9. Dict Comprehension¶

>>> DATA = {'a': 1, 'b': 2, 'c': 3}
>>> result = dict()
>>>
>>> for key, value in DATA.items():
...     result[key] = value

>>> DATA = {'a': 1, 'b': 2, 'c': 3}
>>> result = {k:v for k,v in DATA.items()}

4.14.10. Map¶

>>> def func(x):
...     return float()
...
>>> DATA = [1, 2, 3]
>>> result = (func(x) for x in DATA)

>>> def func(x):
...     return float()
...
>>> DATA = [1, 2, 3]
>>> result = map(func, DATA)

4.14.11. Filter¶

>>> def func(x):
...     return x % 2 == 0
...
>>> DATA = [1, 2, 3]
>>> result = (x for x in DATA if func(x))

>>> def func(x):
...     return x % 2 == 0
...
>>> DATA = [1, 2, 3]
>>> result = filter(func, DATA)

4.14.12. For Else¶

>>> DATA = [1, 2, 3]
>>> FIND = 10
>>> found = False
>>>
>>> for value in DATA:
...     if value == FIND:
...         print('Found')
...         found = True
...         break
...
>>> if not found:
...     print('Not Found')
Not Found

>>> DATA = [1, 2, 3]
>>> FIND = 10
>>>
>>> for value in DATA:
...     if value == FIND:
...         print('Found')
...         break
... else:
...     print('Not Found')
Not Found

4.14.13. While Else¶

>>> DATA = [1, 2, 3]
>>> FIND = 10
>>> found = False
>>>
>>> while i < len(DATA):
...     value = DATA[i]
...     i += 1
...     if value == FIND:
...         print('Found')
...         found = True
...         break
...
>>> if not found:
...     print('Not Found')
Not Found

>>> DATA = [1, 2, 3]
>>> FIND = 10
>>>
>>> while i < len(DATA):
...     value = DATA[i]
...     i += 1
...     if value == FIND:
...         print('Found')
...         break
... else:
...     print('Not Found')
Not Found

4.14.14. Str Startswith¶

>>> data = 'virginica'
>>> data[:1] == 'v'
True
>>> data[:3] == 'vir' or data[:2] == 've'
True

>>> data = 'virginica'
>>> data.startswith('v')
True
>>> data.startswith(('vir', 've'))
True

4.14.15. Str Endswith¶

>>> data = 'virginica'
>>> data[-3:] == 'osa'
False
>>> data[-3:] == 'osa' or data[-2:] == 'ca'
True

>>> data = 'setosa'
>>> data.endswith('osa')
True
>>> data.endswith(('osa', 'ca'))
True

4.14.16. Str Join Newline¶

>>> data = ['line1', 'line2', 'line3']
>>> result = [line+'\n' for line in data]

>>> data = ['line1', 'line2', 'line3']
>>> result = '\n'.join(data)

4.14.17. Others¶

all()
any()
iter()
next()

4.14.18. Functools¶

https://docs.python.org/3/library/functools.html
from functools import *
functools.reduce(function, iterable[, initializer])

4.14.19. Itertools¶

https://docs.python.org/3/library/itertools.html
More information in Itertools
itertools.from itertools import *
itertools.count(start=0, step=1)
itertools.cycle(iterable)
itertools.repeat(object[, times])
itertools.accumulate(iterable[, func, *, initial=None])
itertools.chain(*iterables)
itertools.compress(data, selectors)
itertools.islice(iterable, start, stop[, step])
itertools.starmap(function, iterable)
itertools.product(*iterables, repeat=1)
itertools.permutations(iterable, r=None)
itertools.combinations(iterable, r)
itertools.combinations_with_replacement(iterable, r)
itertools.groupby(iterable, key=None)