Modulus Operator

Check whether a number is divisible by another number using the modulus operator, %

n % k == 0

Open a file

f = open('text.txt','r')
l = f.readlines()
f.close()
for i in l:
    print(i.strip('\n'))

Read Log File

Better way to open a file as it auto closed file, even on crash

with open('/var/log/apache2/access.log','r') as file:
    for line in file:
        print(line.split()[3], line.split()[6], line.split()[8], line.split()[9])
        try:
            b = b +int(line.split()[9])
        except:
            pass

print(b)

List Comprehension

[x**2 for x in range(1,11) if x % 2 == 1]
[x for x in range(1,11) if x % 2 == 0]

Slicing Operator

[start : stop : steps]

Floor Division

Floor Division operator used for dividing two operands with the result as quotient showing only digits before the decimal point:

10//5 = 2
10.0//5.0 = 2.0
10.0//4 = 2.0
10.0//7 = 1.0

List vs. Tuple

List is mutable while the Tuple is not.
A tuple is allowed to be hashed. E.g: using it as a key for dictionaries.

List:  a = [1,3,6]
Tuple: b = (1,4,8)

Lambda vs. def

Def can hold multiple expressions while lambda is a uni-expression function.
Def generates a function and designates a name to call it later. Lambda forms a function object and returns it.
Def can have a return statement. Lambda can’t have return statements.
Lambda supports to get used inside a list and dictionary.

Optional statements in try-except block

Optional statements possible inside a try-except block:

“else” clause: It is useful if you want to run a piece of code when the try block doesn’t create an exception.
“finally” clause: It is useful when you want to execute some steps which run, irrespective of whether there occurs an exception or not.

List Operations

Insert integer at position 5 at position 0:

a.insert(0,5)

Delete the first occurrence of integer:

a.remove(6)

Insert integer at the end of the list:

a.append(9)

Sort the list:

a.sort()

Pop the last element from the list:

 a.pop()

Pop 3rd element:

 list1.pop(2)

Reverse the list:

a.reverse()

Max of List(print runner-ups score):

arr = [1,4,2,6,7,6]
mx = max(arr)
b = [j for j in arr if j != mx]
print(max(b))

List to Tuple Conv & Hashing

>>> a = [1,2,3,45,6,7]
>>> a
[1, 2, 3, 45, 6, 7]

>>> tuple(a)
(1, 2, 3, 45, 6, 7)

>>> hash(tuple(a))
1409902629973635913

Eval Function

You are given a polynomial  of a single indeterminate (or variable), . 
You are also given the values of  and . Your task is to verify if .

Input:

1 4
x**3 + x**2 + x + 1

a = (input()).split()
x = int(a[0])
k = int(a[1])

# Sol 1:
print (eval(input()) == k)

# Python 2:
print eval(raw_input()) == k


# Sol 2:
f = lambda x:eval(input())
print (f(x) == k)

Swap Case

Python has built-in string validation methods for basic data. It can check if a string is composed of alphabetical characters, alphanumeric characters, digits, etc.

str.isalnum() checks if all the characters of a string are alphanumeric (a-z, A-Z and 0-9).

>>> print 'ab123'.isalnum()
True

str.isalpha() checks if all the characters of a string are alphabetical (a-z and A-Z).

>>> print 'abcD'.isalpha()
True

str.isdigit() checks if all the characters of a string are digits (0-9).

>>> print '1234'.isdigit()
True

str.islower() checks if all the characters of a string are lowercase characters (a-z).

>>> print 'abcd123#'.islower()
True

str.isupper() checks if all the characters of a string are uppercase characters (A-Z).

>>> print 'ABCD123#'.isupper()
True

def swap_case(s):
    a = ""
    for i in s:
      if i.isupper():
        a += i.lower()
      elif i.islower:
        a += i.upper()
      else:
        a += i
    return a

if __name__ == '__main__':
    s = input()
    result = swap_case(s)
    print(result)

print ''.join([i.lower() if i.isupper() else i.upper() for i in raw_input()])

def swap_case(s):
    a = ""
    for let in s:
        if let.isupper() == True:
            a+=(let.lower())
        else:
            a+=(let.upper())
    return a
    

 print raw_input().swapcase() 
 print(input().swapcase())

Arrays & Reverse it

>>> input ="1 2 3 4 -8 -10"

>>> arr = input.split(' ')
>>> arr
['1', '2', '3', '4', '-8', '-10']

>>> arr[::-1]
['-10', '-8', '4', '3', '2', '1']

Regex

Positive lookahead     (?=REGEX_1)REGEX_2     (?=regex)      t(?=s) matches the second t in streets.
Negative lookahead     (?!REGEX_1)REGEX_2     (?!regex)      t(?!s) matches the first t in streets.
Positive lookbehind                           (?<=regex)     (?<=s)t matches the first t in streets.
Negative lookbehind                           (?<!regex)     (?<!s)t matches the second t in streets.

Sum Function

lst = []
num = int(input('How many numbers: '))
for n in range(num):
    numbers = int(input('Enter number '))
    lst.append(numbers)
print("Sum of elements in given list is :", sum(lst))

Convert Numeral System

for i in range(0,10):
    print(i, oct(i), hex(i), bin(i))

for i in range(0,10):
    print(i, oct(i)[2:], hex(i)[2:].upper(), bin(i)[2:])

Justify Text

text.rjust(3,' ')
text.ljust(3,' ')
text.center(3,' ')

Formatting

pyformat.info

Old Method:

print('%s %s' % ('one','two'))

New Method:

print('{} {}'.format('one','two'))

Positional Index:

print('{1} {0}'.format(56,'ld'))

Sorting

listed ={}

plist = set(port_list)
listed.update({v:sorted(plist)})

for k,v in listed.items():
    print("Tenant: ", k, "\nNo of Ports: ", len(v), '\n', [i for i in v if len(v) >0], '\n')

c = [1,5,2,3,9,6,2,0]
c.sort()

Reverse a String

'hello world'[::-1]
'dlrow olleh'

Convert to Positive Number

>>> a = -30
>>> abs(a)
30

Interger can be positive or Negative:

>>> int(a)
-30
>>> -a
30

>>> c = 30.9
>>> int(c)
30

"is" vs "=="

a = [1,2,3]
b = a

c = [1,2,3]

print(a==c)
print(a is c)

Output:
True
False

Dictionary

a = {}