2.9. builtin - Logic Loops¶

There are infinite ways to write code but it is important to highlight the wise words from The Zen of Python: “Flat is better than nested” in this section more than any other because it is far too easy to nest logic loops that are confusing to read after numerous indents. As a suggestion try to limit nested loops to 3-4; not only will it help downstream users pickup your code easier but the code will also be easier to test. Logic that needs more than 3-4 nested loops should be broken up into separate functions.

2.9.1. True False¶

In python the following all evaluate to True:

0 == True (note that float 0.0 is not True, only the int 0)
‘any non empty str’ == True
non empty set, tuple, list, dict

2.9.2. General “and” “or” “not” “any” “all”¶

2 == 5 or 10 == 10
>>> True

2 == 5 and 10 == 10
>>> False

2 != 5 and 10 == 10
>>> True
# in a alternate form
not 2 == 5 and 10 == 10
>>> True

# "any" is powerful with list comprehensions:
any(i == 4 for i in [3,4,5])
>>> True
any(i == 10 for i in [3,4,5])
>>> False

# "all" works similar to "any" but all instances of the iterable must eval to True
all(i%2 == 0 for i in [4,6,8])
>>> True # because all num/2 result with a remainder of 0 (the values are all even)
all(i%2 == 0 for i in [4,5,8])
>>> False # this evals to True, False, True which is overall False

# there is one more that can be imported from builtin library "operator"
from operator import xor
# xor: one or the other true but not both
1 == 1 xor 2 == 1
>>> True

2.9.3. if elif else¶

x = 5
if x == 5:
    print('x == 5')
elif x == 6:
    print('x != 5 but x does equal 6!')
elif x == 7:
    print('x !=5, x != 6, but x == 7')
else:
    print('x was not equal to 5, 6, or 7')

# there is a 1 liner short for a simple if/else
y = x + 10 if x == 5 else 0
>>> y = 15 # these are really useful for initializing variable

2.9.4. for loop¶

# iterate through strings by char
for char in "this":
    print(char)
>>> 't'
>>> 'h'
>>> 'i'
>>> 's'

for value in range(start=1, stop=30, step=10):
    print(value)
>>> '1'
>>> '11'
>>> '21'

# iterate through sets, tuples, lists
for value in [10,20,30]:
    print(value)
>>> '10'
>>> '20'
>>> '30'

# it is often useful to iterate through the values and also keep index
for index, value in enumerate([10,20,30], start=100):
    print(index, value)
>>> '100 10'
>>> '101 20'
>>> '102 30'

# iterate through dicts (iterate on keys, values, or items)
for key, value in {'key1':1, 'key2':2}.items():
    print(key, value)
>>> 'key1 1'
>>> 'key2 2'

# for loop on multiple same same iterators
for val1, val2 in zip([1,2,3],[10,20,30]):
    print(val1,val2)
>>> '1 10'
>>> '2 20'
>>> '3 30'

# use break to jump out of a for loop early
for val in [1,2,3]:
    if val == 2:
        break
    print(val)
>>> '1'
# but never gets to printing 2 or 3

# use continue to jump ahead of the current iteration (same as a __next__() call)
for val in [1,2,3]:
    if val == 2:
        continue
    print(val)
>>> '1'
>>> '3'
# note how 2 was skipped

2.9.5. List Comprehensions (alt for loops)¶

# a simple for loop
vals = []
for value in colletion:
    if condition:
        vals.append(expression)

# can be written in 1 line with list comprehension
vals = [expression for value in collection if condition]

example

vals = []
for value in [1,2,3]:
    if value%2 == 1:
        vals.append(value + 10)

vals
>>> [11,13]


# now with list comprehension
vals = [value + 10 for value in [1,2,3] if value%2 == 1]

vals
>>> [11,13]

similarly dictionaries can also be handled with list comprehensions

vals = ["/".join(key, str(value)) for key, value in {'one': 1, 'two': 2}.items()]

vals
>>> ['one/1', 'two/2']

dictionary comprehensions

vals = {k: 2*v for k, v in {'one': 1, 'two': 2}.items()}

vals
>>> {'one': 2, 'two': 4}

2.9.6. while loop¶

i == 0
while i < 3:
    print(i)
    i += 1
else:
    'while loop finished without a break'
>>> '1'
>>> '2'
>>> '3'
>>> 'while loop finished without a break'

i == 0
while i < 3:
    print(i)
    if i == 2:
        print('while loop finished early with a break')
        break
    i += 1
else:
    'while loop finished without a break'
>>> '1'
>>> '2'
>>> 'while loop finished early with a break'

2.9.7. try/except/pass¶

See full list of exception at Link

try:
    # somecode to test for exceptions
except NameError:
    # somecode raised a NameError, do something
except (ValueError,KeyError):
    # samecode did not raise a NameError, but it did raise either
    #   a ValueError or KeyError, do something
except:
    # catch all other errors, this is lazy coding - try to not use this
    #   the owner should understand what exceptions occur and handle it appropriately
else:
    # no exception were raised, do something
finally:
    # run code lastly before exiting try loop, no matter if an exception was or not

2.9.8. Trick - Type check multiple variables at the same time¶

# "all" is a check that all argument are True inside
# isinstance checks if an object is made of some parent object (ie, isinstance("this", str) == True)
# the rest is a list comprehension to cycle through multiple objects on the same line
if not all(isinstance(variable, (int, float) for variable in [a, b, c, d])):
    raise TypeError("Incorrect variable type entry, a,b,c,d must be either int or float")