Loops and conditions¶

if ... else¶

if expression: 
  statement(s)

if expression: 
  statement(s)
else: 
  statement(s)

if expression1: 
  statement(s)
elif expression2: 
  statement(s)
else: 
  statement(s)

Single line if else.¶

'Yes' if fruit == 'Apple' else 'No'
a = 10 if fruit == 'Apple' else 20
if a == 20: print(a)

For loop¶

for something in somethings:
   do something

Single Line for:¶

doubled = [thing for thing in list_of_things]
images = [image.id for image in ec2.instances.all()]

Iterating two lists using `for` loop using `zip`:¶

zip Iterate two lists in parallel

a = ['abc', 'xyz']
b = ['123' '456']
for x,y in zip(a, b):
  print(x,y)
...
('abc', '123')
('xzy', '456')

Iterate over two lists and their indices using enumerate together with zip¶

alist = ['a1', 'a2', 'a3']
blist = ['b1', 'b2', 'b3']

for i, (a, b) in enumerate(zip(alist, blist)):
    print i, a, b

0 a1 b1
1 a2 b2
2 a3 b3

While Loop¶

while true:
   do something

While with 2 variables¶

while len(list1) and len(list2):
  if list1[0] < list2[0]:

Break, continue and pass¶

break : breaks the ‘for’ or ‘while’ loop.
continue: continues with the next iteration of the loop.
pass: It does nothin. It is used when a statement is required syntactically but requires no action.

>>> raise ValueError('Hi there')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Hi there

Try, Except, Else and Finally¶

If an error (exception) is encountered, a try block code execution is stopped and transferred down to the except block.
In addition to using an except block after the try block, you can also use the finally block.
The code in the finally block will be executed regardless of whether an exception occurs.

>>> try:
...    print("helo world")
... except:
...    print("here goes the error”
... finally:
        print(all done)

If exception type is given after the except keyword, and the the exception code matches, then the except clause is executed.

>>> try:
...    print('if variable VAR1 id defined')
...    VAR1
... except NameError:
...    print('variable VAR1 is not defined')
...
if variable VAR1 id defined
variable VAR1 is not defined

The try ... except statement has an optional else clause, which, when present, must follow all except clauses. It is useful for code that must be executed if the try clause does not raise an exception. Eg.

for arg in sys.argv[1:]:
    try:
        f = open(arg, 'r')
    except IOError:
        print 'cannot open', arg
    else:
        print arg, 'has', len(f.readlines()), 'lines'
        f.close()