Python essentials

List comprehensions

• A list comprehension is a compact way to write an expression that expands to a whole list.
• List comprehension can almost substitute for the lambda function especially as map(), filter() and reduce().

Examples

# compute a list of their squares of elements in a list
nums = [1, 2, 3, 4]

squares = [ n * n for n in nums ]   ## [1, 4, 9, 16]

>>> port_numbers = [22,25,80,139,443]
>>> [k for k in port_numbers]
[22, 25, 80, 139, 443]
>>>
>>> numbers = [2,4,6,8] 
>>> [k*2 for k in numbers]
[4, 8, 12, 16]

• You can add an if test to the right of the for-loop to narrow the result. The if test is evaluated for each element, including only the elements where the test is true.

>>> port_numbers = [22,25,80,139,443,8080,9999,55555]
>>> 
>>> [k for k in port_numbers if k<1023]
[22, 25, 80, 139, 443]

Lambda functions

Syntax: lambda arg1, arg2, …argN : expression using arguments.

- Python supports the creation of anonymous functions (i.e. functions that are not bound to a name) at runtime, using a construct called “lambda”.
- Lambda functions take any number of arguments but only has one expression, the result of this expression is returned.

>>> def f (x): return x**2
... 
>>> print f(8)
64
>>> 
>>> g = lambda x: x**2
>>> 
>>> print g(8)
64

• Lambda functions can be used anywhere a function is expected, not necessary to assign it to a variable.
  
• You can pass lambda functions to other functions to do stuff(for example lambda function as key to sorted())
  

• Extracting numbers that are multiples of 3.
• One element of data structure(a list here) gets passed to the lambda function at a time.
• Lambda function can only have one expression.
• filter is a built-in function in Python that take a function and a data structure as arguments, passes one element of data structure at a time to the function.

>>> mult3 = filter(lambda x: x % 3 == 0, [1, 2, 3, 4, 5, 6, 7, 8, 9])
>>> mult3
[3, 6, 9]

• Sort elements in list by last character.

>>> lis = ['az', 'bbbbbbt', 'cccx']
>>>
>>> sorted_by_last = sorted(lis, key = lambda(x): x[-1] )   
>>>
>>> sorted_by_last
['bbbbbbt', 'cccx', 'az']

• Extracting privileged ports(under 1024) from a list of port numbers using lambda function.

>>> open_ports = [1,20,21,25,80,443,1639,4444,6798]
>>> 
>>> privileged_ports = filter(lambda x: x <1024, open_ports)
>>> privileged_ports
[1, 20, 21, 25, 80, 443]
>>> 

• Doing the same with normal function.
• If you are not comfortable with using lambda functions, you can simply use normal functions.

>>> def extract_priv_ports(port_number):
...     if port_number < 1024:
...         return port_number
>>>
>>> privileged_ports = filter(extract_priv_ports, open_ports)
>>> privileged_ports
[1, 20, 21, 25, 80, 443]
>>>