Coding Questions II - 2024

In information theory, the Hamming distance between two strings of equal length is the number of positions at which the corresponding symbols are different:

Here is the code for the Hamming distance:

def hamming(s1,s2): 
    if len(s1) != len(s2): 
        raise ValueError("Not defined - unequal lenght sequences") 
    return sum(c1 != c2 for c1,c2 in zip(s1,s2))

if __name__ == '__main__': 
    print(hamming("karolin", "kathrin")) # 3 
    print(hamming("karolin", "kerstin")) # 3 
    print(hamming("1011101", "1001001")) # 2
    print(hamming("2173896", "2233796")) # 3
    print(hamming("00000", "11111")) # 5

The first zip will produce [('k', 'k'), ('a', 'a'), ('r', 't'), ('o', 'h'), ('l', 'r'), ('i', 'i'), ('n', 'n')]. and the sum() counts the number of False, for example 3, because the input for sum() should look like this: [False, False, True, True, True, False, False].

Write a floor division function without using '/' or '%' operator. For example, f(5,2) = 5/2 = 2, f(-5,2) = -5/2 = -3.

def floor(a,b):
   count = 0
   sign = 1
   if a < 0: sign = -1
   while True:
      if b == 1: return a
      # positive
      if a >= 0:
         a -= b
         if a < 0: break
      # negative
      else:
         a = -a - b
         a = -a
         if a > 0:
            count += 1
            break
      count += 1
   return count*sign

from random import randint
n = 20
while n > 0:
  a, b = randint(-20,20), randint(1,10)
  print '%s/%s = %s' %(a,b, floor(a,b))
  n -= 1

Output:

11/4 = 2
3/5 = 0
11/5 = 2
10/1 = 10
-6/9 = -1
14/2 = 7
-4/7 = -1
8/6 = 1
7/1 = 7
-7/1 = -7
-20/5 = -4
11/4 = 2
16/10 = 1
-9/7 = -2
-7/6 = -2
-8/2 = -4
13/7 = 1
-3/2 = -2
-10/2 = -5
-6/1 = -6

Q1: How do you fetch every other item in the list?

Suppose we have a list like this:

>>> L = [x*10 for x in range(10)]
>>> L
[0, 10, 20, 30, 40, 50, 60, 70, 80, 90]

1. We can use enumerate(L) to get indexed series:

   >>> for i,v in enumerate(L):
   ...    if i % 2 == 0:
   ...       print v,
   ... 
   0 20 40 60 80
   

2. This one may be the simplest solution:

   L2 = L[::2]
   >>> L2
   [0, 20, 40, 60, 80]
   

What's get printed?

def f(): 
   pass
print type(f())
print type(1J)
print 1j*1j
print type(lambda:None)

Answer:

<type 'NoneType'>
<type 'complex'>
(-1+0j)
<type 'function'>

1. The argument to the type() call is a return value of a function call, which returns None.
2. 'lambda arguments: expression' yields a function object.
3. An imaginary literal yields a complex number with a real part of 0.0.

Q2: What's the list comprehension? How about dictionary comprehension?
Construct x^3 from a list of integers, L = [0,1,2,3,4,5]

List comprehension:

>>> L2 = [x**3 for x in L]
>>> L2
[0, 1, 8, 27, 64, 125]

Or:

>>> L2 = [x**3 for x in range(6)]
>>> L2
[0, 1, 8, 27, 64, 125]

Dictionary comprehension:

>>> D1 = dict((k,k**3) for k in L)
>>> D1
{0: 0, 1: 1, 2: 8, 3: 27, 4: 64, 5: 125}

Or for Python 3:

>>> D2 = {k:k**3 for k in L}
>>> D2
{1: 1, 2: 8, 3: 27}

Q3: Sum of all elements in a list ([1,2,3,...,100] with one line of code.

>>> Ans = sum(range(101))
>>> Ans
5050

Q3.b: How about summing only odd elements?

>>> sum(range(1,101,2))

Or:

>>> Ans = sum(x for x in range(101) if x % 2 != 0)
>>> Ans
2500

Or:

>>> Ans = reduce(lambda x,y: x+y, filter(lambda x: x % 2 != 0, range(1,101)))
>>> Ans
2500

Or

>>> sum(filter(lambda x: x % 2, range(1,101)))

Two divisions - '/' and '//':

>>> 5.5/2
2.75
>>> 5.5//2
2.0

Q: What are the differences between Python 2 and Python 3?

Please visit Python 2 vs Python 3.

What's the output?

>>> len(set([1,1,2,3,3,3,4]))

Output:

4

set() only retains unique values.

We want to print a list of files in a directory including the sub-directories. We may want to do it recursively.

import os

def file_list(dir):
    basedir = dir
    subdir_list = []
    for item in os.listdir(dir):
        fullpath = os.path.join(basedir,item)
        if os.path.isdir(fullpath):
            subdir_list.append(fullpath)
        else:
            print fullpath

    for d in subdir_list:
        file_list(d)

file_list('/dir')

Output:

./d3/f1
./d3/d4/d7/f2
./d3/d4/d7/f3
./d3/d5/d9/d10/f4
./d3/d5/d9/d10/f5

Here is simpler code:

import os

path = './d3'
file_list = []
for root, dir_names, file_names in os.walk(path):
    for f in file_names:
        file_list.append(os.path.join(root,f))
print(file_list)

Output:

['./d3/f1', './d3/d4/d7/f3', './d3/d4/d7/f2', './d3/d5/d9/d10/f4', './d3/d5/d9/d10/f5']

For a given sentence, for example, "The Mississippi River", count the occurrence of a character in the string.

sentence='The Mississippi River'

def count_chars(s):
        s=s.lower()
        count=list(map(s.count,s))
        return (max(count))

print count_chars(sentence)

Let's look into the code, count=list(map(s.count,s))

>>> count = map(s.count,s)
>>> count
[1, 1, 2, 2, 1, 5, 4, 4, 5, 4, 4, 5, 2, 2, 5, 2, 1, 5, 1, 2, 1]

Actually, the each item in the list output represents the occurrence of each character including space:

 T  h  e     M  i  s  s  i  s  s  i  p  p  i     R  i  v  e  r
[1, 1, 2, 2, 1, 5, 4, 4, 5, 4, 4, 5, 2, 2, 5, 2, 1, 5, 1, 2, 1]

Answer = 5

In the line, map(func, sequence), the s.count counts the occurrence of a character while it iterates the character sequence of s.

Or

sentence='The Mississippi River'

def count_chars(s):
    s=s.lower()
    L = [s.count(c) for c in s]
    return max(L)

print (count_chars(sentence))

Or we can use dictionary:

sentence='The Mississippi River'
sl = sentence.lower()

d = {}.fromkeys([x for x in sl],0)

for char in sl:
    d[char] += 1
    
print d

Output:

{' ': 2, 'e': 2, 'i': 5, 'h': 1, 'm': 1, 'p': 2, 's': 4, 'r': 2, 't': 1, 
'v': 1} 

Or we can use dict.get(key,None) method:

sentence='The Mississippi River'
sl = sentence.lower()
d = {}
for c in sl:
    d[c] = d.get(c,0) + 1  
print d

Output:

{' ': 2, 'e': 2, 'i': 5, 'h': 1, 'm': 1, 'p': 2, 's': 4, 'r': 2, 't': 1, 'v': 1}

Or we can use dictionary comprehension:

sentence = "The Mississippi River"
ans = dict((c,sentence.count(c)) for c in sentence)
print ans

Output:

{' ': 2, 'e': 2, 'i': 5, 'h': 1, 'M': 1, 'p': 2, 's': 4, 'R': 1, 'T': 1, 'v': 1, 'r': 1}

import math
def isPrime(n):
    if n <= 1:
        return False
    if n == 2:
        return True
    if n % 2 == 0:
        return False
    for t in range(3, int(math.sqrt(n)+1),2):
        if n % t == 0:
            return False
    return True

print [n for n in range(100) if isPrime(n)]

Output:

[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97]

A simpler code:

def isPrime(n):
   if n == 1:
      return False
   for t in range(2,n):
      if n % t == 0:
         return False
   return True

print [n for n in range(1,100) if isPrime(n)]

Not much but a little bit of tweak: generate a list with the first 100 primes:

import math
def isPrime(n):
   if n == 1:
      return False
   elif n == 2:
      return True
   elif n % 2 == 0:
      return False
   else:
      for t in range(3, int(math.sqrt(n)+1), 2):
         if ( n % t == 0 ):
            return False
   return True

count = 1
n = 1
primes = []
while (count <= 100):
   if isPrime(n):
      primes.append(n)
      count += 1
   n += 1

print primes

Run it:

$ python prime2.py
python prime.py
[2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271, 277, 281, 283, 293, 307, 311, 313, 317, 331, 337, 347, 349, 353, 359, 367, 373, 379, 383, 389, 397, 401, 409, 419, 421, 431, 433, 439, 443, 449, 457, 461, 463, 467, 479, 487, 491, 499, 503, 509, 521, 523, 541]

def isPrime(n):
    if n == 1: 
        return False
    if n == 2:
        return True
    for i in range(2,n):
        if n % i == 0:
            return False   
    return True
      
MaxCount = 10 
c = 0  
n = 2
primes = [] 
while (c <= MaxCount):
    if isPrime(n):
        primes.append(n)
        c += 1
    n += 1    

Output for MaxCount = 10:

primes = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31]   

When it comes to reversing a string in python, we do have s[::-1], boring!

In the following recursive code, at every call, the first string will go to the end and stored in stack. So, when there is only one character left as an input string, the code starts rewind and retrieve the character one by one. The net effect is it gets the character in reverse. It becomes obvious when we see the printed output below:

def reverse(input):
    print input
    if len(input) <= 1:
	    return input
	
    return reverse(input[1:]) + input[0]

s = 'reverse'	
print(reverse(s))

Output:

reverse
everse
verse
erse
rse
se
e
esrever

See also Reversing words

In the code below, we use range index operation, putting the characters into the list in reverse order. Than we pack them using join():

def reverse(input):
    return ''.join([input[i] for i in range(len(input)-1, -1, -1)])
    # or return ''.join(input[-i-1] for i in range(len(input)))
    # or return input[::-1]
s = 'reverse'	
print(reverse(s))

In the range() function, we used "-1" as a "stop" index and another "-1" as a "step" size.

Output:

esrever

See also Reversing words

Return a given Number say 12345, in reverse order. So, it should return 54321:

# operates on a number directly
def rev1(n):
  r = 0
  for i in range(len(str(n))):
     r += n % 10
     n = n / 10
     # promote digits except the last digit
     if (n > 0):
       r = r * 10
  return r

# operates on a converted string
def rev2(n):
   sn = str(n)
   r = ''.join(sn[-i-1] for i in range(len(sn)))
   return r

# operates on a converted string - simplest
def rev3(n):
   return str(n)[::-1]

n = 12345
print(rev1(n))
print(rev2(n))
print(rev3(n))    

Output:

54321
54321
54321    

Q: What's the output from the following code?

def f():
    yield

def g():
    return

print 'f()=', f()
print 'g()=', g()

You can find the answer from generator.send() method.

Q: We have a list of tuples where each tuple is a (start, end). We want to merge all overlapping ranges and return a list of distinct ranges. For example:

[(1, 5), (2, 4), (3, 6)] --->  [(1, 6)]
[(1, 3), (2, 4), (5, 6)] --->  [(1, 4), (5, 6)]

Write a code to produce a list of tuples with distinct ranges. Try with this data:

L = [(4,9), (20, 22), (1, 3), (24, 32), (23, 31), (40,50), (12, 15), (8,13)]

Here is the code:

L = [(4,9), (20, 22), (1, 3), (24, 32), (23, 31), (40,50), (12, 15), (8,13)]
# L = [(1, 5), (2, 4), (3, 6)]
# L = [(1, 3), (2, 4), (5, 6)]
L = sorted(L)
print L

Lnew = []
st = L[0][0]
end = L[0][1]

for item in L[1:]:
    if end >= item[0]:
        if end < item[1]:
            end = item[1]
    else:
        Lnew.append((st,end))
        st = item[0]
        end = item[1]
    
Lnew.append((st,end))

print Lnew

Output:

[(1, 3), (4, 9), (8, 13), (12, 15), (20, 22), (23, 31), (24, 32), (40, 50)]
[(1, 3), (4, 15), (20, 22), (23, 32), (40, 50)]

Q: Outputs the result using ternary operator. 'pass' when the 'expected' is the same as 'returned', otherwise outputs 'fail'. Assume 'expected' = 1, 'returned' = 0.

expected = 1
returned = 0
result = 'pass' if expected == returned else 'fail'
print result

In Python, packing and unpacking refer to operations involving the grouping and unpacking of values in data structures. This often involves tuples, lists, and iterable objects. Let's look at both concepts:

1. Packing: the process of combining multiple values into a single variable or data structure. Tuples are commonly used for packing

   # Packing values into a tuple
   packed_tuple = 1, 'apple', 3.14
   
   # Packed tuple
   print(packed_tuple)
   # Output: (1, 'apple', 3.14)         
   



2. Unpacking: the reverse process of extracting values from a data structure (like a tuple or a list) and assigning them to individual variables.

   # Packing values into a tuple
   packed_tuple = 1, 'apple', 3.14
   
   # Packed tuple
   print(packed_tuple)
   # Output: (1, 'apple', 3.14)         
   

   
   We can also use the * operator for variable-length unpacking, where one variable gets assigned the remaining elements as a list:

   packed_tuple = 1, 'apple', 3.14
       
   first, *rest = packed_tuple
   
   print(first)
   # Output: 1
   
   print(rest)
   # Output: ['apple', 3.14]    
   



3. Packing and Unpacking in Function Arguments: Packing and unpacking are often used in function arguments.

   # Packing arguments into a tuple
   def example_function(*args):
       print(args)
   
   example_function(1, 'apple', 3.14)
   # Output: (1, 'apple', 3.14)       
   
   # Unpacking arguments from a tuple
   def another_function(a, b, c):
       print(a, b, c)
   
   arguments = (1, 'apple', 3.14)
   another_function(*arguments)
   # Output: 1 apple 3.14
   

   In this example, the *arguments syntax unpacks the values from the tuple and passes them as separate arguments to the function. Packing and unpacking are versatile techniques that enhance the flexibility and readability of our code, especially when working with functions or dealing with multiple values.

What will be printed?

def getThem(arg1, *arg2):
   print type(arg2), arg2

getThem('uno','dos','tres','cuatro','cinco')

arg2 aggregates the rest of args into a tuple. So, the output should look like this:

<type 'tuple'> ('dos', 'tres', 'cuatro', 'cinco')

How about this one:

def getThem(arg1, **arg2):
   print type(arg2), arg2

getThem('numbers',one='uno',two='dos',three='tres',four='cuatro',five='cinco')

arg2 aggregates the remaining parameters into a dictionary. So, the output is:

<type 'dict'> {'four': 'cuatro', 'three': 'tres', 'five': 'cinco', 'two': 'dos', 'one': 'uno'}

Will the code below work?

def getThem(a,b,c,d,e):
   print a,b,c,d,e

mList = ['uno','dos','tres','cuatro','cinco']
getThem(mList)

Answer: No.

We'll get an error:

TypeError: getThem() takes exactly 5 arguments (1 given)

So, the code should be changed like this:

getThem(*mList)

*mList will unpack the list into individual elements to be passed to the function.

The unpacking a list can also be used as this:

>>> a, *b, c = [1,2,3,4,5,6,7,8,9,10]
>>> print(b)
[2, 3, 4, 5, 6, 7, 8, 9]

>>> a, *b, c, d = [1,2,3,4,5,6,7,8,9,10]
>>> print(b)
[2, 3, 4, 5, 6, 7, 8]

We have (oth-7th) revisions committed to Github, and due to a testing flaw, at the 7th revision we found that a bug has been introduced. We want to find when it was committed:

# r0 r1 r2 r3 r4 r5 r6 r7
# G  ?  ?  ?  ?  ?  ?  B   (G: good, B: buggy)

We may want to use binary search: if found bug, move backwards (previous rev.), if not, mode forwards (later rev.). In the code sample below, we're testing 6 cases:

def hasBug(i):
   if rev[i] == 'B':
      return True
   return False

def find_bug(good, bad):
   st = good
   end = bad
   ret = -1
   mid = (st + end) / 2
   while (mid > good and mid < bad):
      # backwards
      if hasBug(mid):
         end = mid
         ret = mid
         if mid == good + 1: break
      # forwards
      else:
         st = mid
         if mid == end - 1: break
      mid = (st + end ) / 2

   return ret

if __name__ == "__main__":
   rev = ['G','B','B','B','B','B','B','B']
   print 1 == find_bug(0,7)

   rev = ['G','G','B','B','B','B','B','B']
   print 2 == find_bug(0,7)

   rev = ['G','G','G','B','B','B','B','B']
   print 3 == find_bug(0,7)

   rev = ['G','G','G','G','B','B','B','B']
   print 4 == find_bug(0,7)

   rev = ['G','G','G','G','G','B','B','B']
   print 5 == find_bug(0,7)

   rev = ['G','G','G','G','G','G','B','B']
   print 6 == find_bug(0,7)

Note that the find_bug() function returns the first revision # that introduced a bug.

Output:

True
True
True
True

What will be printed out?

>>> True or False and False

Since AND is higher precedence than OR, AND will be evaluated first, "True" will be printed out.

Then, how about this one?

>>> not True or False or not False and True
True

NOT has first precedence, then AND, then OR.

We have a function that returns the amount with tax added. We want to prefix the amount with a dollar($) using decorator:

@dollar
def price(amount, tax_rate):
    return amount + amount*tax_rate

How do we want to implement the decorator function, dollar()?

Here is the code:

def dollar(fn):
    def new(*args):
        return '$' + str(fn(*args))
    return new

@dollar
def price(amount, tax_rate):
    return amount + amount*tax_rate

print price(100,0.1)

Output:

$110

Note that we did not modify the decorated function, price(), at all.

This example is not much different from the previous one.

We have a function returning squared number:

def oldFnc(n):
   return n*n

Write a code doubling the return value from oldFnc() just by adding a decorator like this:

@doubleIt
def oldFnc(n):
   return n*n

Here is a code:

def doubleIt(myfnc):
   def doubleItInside(*args):
     return myfnc(*args)*2
   return doubleItInside

@doubleIt
def oldFnc(n):
   return n*n

#oldFnc = doubleIt(oldFnc)

print oldFnc(5)

For more information about decorators and the examples, please check Decorators

The following code won't print out properly. What's wrong with the code?

days = ['Monday',
        'Tuesday',
        'Wednesday']

months = ['Jan', \
         'Feb', \
         'Mar']

print "DAYS: %s, MONTHS %s" % 
    (days, months)

days is OK because expressions in parentheses, square brackets or curly braces can be split over more than one physical line without using backslashes. The months is also OK because backslashes are used to join physical lines, even though they are not required in this case. The print statement will not print the data because 2 logical lines are used without a backslash to join them into a logical line.

So, correct code should look like this:

print "DAYS: %s, MONTHS %s" %\ 
    (days, months)

Recursive binary serach:

# returns True if found the item
# otherwise, returns False
def bSearch(a, item):
   if len(a) == 0:
      return False
   mid = len(a)/2
   if a[mid] == item:
      return True
   else:
      if a[mid] < item:
         return bSearch(a[mid+1:], item)
      else:
         return bSearch(a[:mid], item)

if __name__ =="__main__":
   a = [1,3,4,5,8,9,17,22,36,40]
   print bSearch(a, 9)
   print bSearch(a, 10)
   print bSearch(a, 36)

Output:

True
False
True

Iterative binary serach:

# returns True if found the item
# otherwise, returns False
def bSearch(a, item):
   st = 0
   end = len(a)-1
   mid = len(a)/2
   left = st
   right = end
   while True:
      # found
      if a[mid] == item:
         return True
      # upper
      elif a[mid] < item:
         left = mid + 1
      # lower
      else:
         right = mid - 1

      mid = (left+right)/2
      if mid < st or mid > end: break
      if left == right and a[mid] != item: break
   return False

if __name__ =="__main__":
   a = [1,3,4,5,8,9,17,22,36,40]
   print bSearch(a, 9)
   print bSearch(a, 10)
   print bSearch(a, 36)
   print bSearch(a, 5)
   print bSearch(a, 22)

Output:

True
False
True
True
True

List is passed by reference to the function and modifications to the function parameter also effect the original list.

What's the size of the list, (len(mList)).

def addItem(l):
    l += [0]

mList = [1, 2, 3, 4]
addItem(mList)
print len(mList)

Answer: 5

This is not a real calculator. It's a practice of stack implementation by manipulating list's pop() and append().

The code should pass the following test:

test = {"11++":-1, "1+2": -1, "12+":3, "12+4*":12, "12+4*5+":17, "42-5*7-":3}

The keys are the question strings, and the values are the correct answers.

Note the following:

1. We should have at least two items in the list whenever we see '+'/'-'/'*' operators. Otherwise, it should issue -1 and exit.
2. For '-' operation, we need to take care of the the order of pop() operation. In the code, we prepend '-' for the last item in the list and then pops the 2nd to the last after that.

Here is the code:

def sol(input):
   a = []
   print "input=",input
   for c in input:
      if c.isdigit():
         a.append(c)
      elif c == '+':
         if len(a) < 2:
            return -1
         a.append(int(a.pop())+int(a.pop()))
      elif c == '-':
         if len(a) < 2:
            return -1
         a.append(-int(a.pop())+int(a.pop()))
      elif c == '*':
         if len(a) < 2:
            return -1
         a.append(int(a.pop())*int(a.pop()))
      else:
         pass
   return a[0]

test = {"11++":-1, "1+2": -1, "12+":3, "12+4*":12, "12+4*5+":17, "42-5*7-":3}

for k,v in test.items():
   if sol(k) == v:
      print k, v, 'Passed'
      print
   else:
      print k, v, 'Error'
      print

Output:

input= 12+4*
12+4* 12 Passed

input= 42-5*7-
42-5*7- 3 Passed

input= 11++
11++ -1 Passed

input= 12+
12+ 3 Passed

input= 12+4*5+
12+4*5+ 17 Passed

input= 1+2
1+2 -1 Passed

Write a class that returns network interfaces (eth0, wlan0, ...).

Use subprocess and ip link show command.

import subprocess

class interfaces:
   def __init__(self):
      self.intf = []
      proc = subprocess.Popen(['ip', 'link', 'show'], stdout=subprocess.PIPE)
      line_count = 0
      while True:
         line = proc.stdout.readline()
         if line != '':
            if line_count % 2 == 0:
               self.intf.append(line)
         else:
            break
         line_count += 1
      self.counter = -1
      self.max = len(self.intf)

   def __iter__(self):
      return self

   def next(self):
      if self.counter >= self.max-1:
         raise StopIteration
      self.counter += 1
      return self.intf[self.counter]

f = interfaces()
for i in f:
   print i

Note that we used next() instead of __next__() because we're using Python 2.

Output:

1: lo:  mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default 

2: eth0:  mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT group default qlen 1000

3: wlan0:  mtu 1500 qdisc mq state UP mode DORMANT group default qlen 1000

4: lxcbr0:  mtu 1500 qdisc noqueue state UNKNOWN mode DEFAULT group default 

5: docker0:  mtu 1500 qdisc noqueue state DOWN mode DEFAULT group default 

8: vmnet1:  mtu 1500 qdisc pfifo_fast state UNKNOWN mode DEFAULT group default qlen 1000

9: vmnet8:  mtu 1500 qdisc pfifo_fast state UNKNOWN mode DEFAULT group default qlen 1000

We have a file that lists domain names. We want to write a Python function that takes the file and returns a dictionary that has a key for the domain and a value for ip.

For example, the input file ('domains.txt') looks like this:

google.com
yahoo.com

Output is:

{'yahoo.com': '98.139.183.24', 'google.com': '216.58.192.14'}

Hint: we may want to use dig command, and to make the output simpler (only want to get ANSWER section), we can use it with additional options: +noall and +answer.

Here is the code:

import subprocess

def getIPs(file):
   ips = {}
   with open(file) as f:
      for line in f:
         domain = line.strip()
         out = subprocess.check_output(["dig","+noall","+answer", domain])
         ips[domain] = out.split()[4]
   return ips

print getIPs('domains.txt')

We have a class A, and we want to count the number of A instances.

Hint : use staticmethod

class A:
   total = 0

   def __init__(self, name):
     self.name = name
     A.total += 1

   def status():
      print "Total number of instance of A : ", A.total
   status = staticmethod(status)


a1 = A("A1")
a2 = A("A2")
a3 = A("A3")
a4 = A("A4")

A.status()

Output:

Total number of instance of A :  4

We can use decorator (@staticmethod) like this:

   @staticmethod
   def status():
      print "Total number of instance of A : ", A.total

Ref : https://docs.python.org/2/library/profile.html

cProfile and profile provide deterministic profiling of Python programs. A profile is a set of statistics that describes how often and for how long various parts of the program executed.

We can find the explanation of the output from the reference:

In the output from fibo(10), the first line indicates that 21893 calls were monitored. Of those calls, 3 were primitive, meaning that the call was not induced via recursion. The next line: Ordered by: standard name, indicates that the text string in the far right column was used to sort the output. The column headings include:

1. ncalls
   for the number of calls
2. tottime
   for the total time spent in the given function (and excluding time made in calls to sub-functions)
3. percall
   is the quotient of tottime divided by ncalls
4. cumtime
   is the cumulative time spent in this and all subfunctions (from invocation till exit). This figure is accurate even for recursive functions.
5. percall
   is the quotient of cumtime divided by primitive calls
6. filename:lineno(function)
   provides the respective data of each function

When there are two numbers in the first column (for example 21891/1), it means that the function recursed. The second value is the number of primitive calls and the former is the total number of calls. Note that when the function does not recurse, these two values are the same, and only the single figure is printed.

The super() can be used when the root class inherits from the object class, and here is the code of calling a base class method from a child class that overrides it:

A module can find out its own module name by looking at the predefined global variable __name__. If this has the value '__main__', the program is running as a script.

Many modules that are usually used by importing them also provide a command-line interface or a self-test, and only execute this code after checking __name__:

# t.py
def f():
    print(f"module name is {__name__}")

if __name__ == '__main__':
    f()

We have a simple Python module t.py with a function f() and a conditional statement that checks if the module is being run as the main program. If it is, it calls the f() function.

1. The function f() prints the name of the module using the __name__ variable. When a Python script is run, the __name__ variable is set to '__main__'. If the module is imported into another script, __name__ is set to the name of the module.
2. The conditional statement if __name__ == '__main__': ensures that the f() function is only called if the script is executed directly, not if it is imported as a module in another script.
3. If we run this script directly (not imported as a module), it will print "module name is main" to the console. However, if we import it as a module in another script, the f() function won't be called automatically.

Run it as a script directly:

$ python t.py
module name is __main__

If we import the t.py, the module name will be t:

# s.py
import t    

$ python s.py
module name is t

Note that if want the f() to be called when imported the conditionals should be commented out:

# t.py
def f():
    print(f"module name is {__name__}")
    
# if __name__ == '__main__':
#    f()

f()

Now, when we import t in another script, the f() function will be executed automatically. Keep in mind that this approach makes the f() function run every time the module is imported, which may or may not be desired depending on the use case.

We might be wondering why appending an element to newL changed L too.

There are two factors that produce this result:

1. Variables are simply names that refer to objects. Doing newL = L doesn't create a copy of the list - it creates a new variable newL that refers to the same object L refers to. This means that there is only one object (the list), and both L and newL refer to it.
2. Lists are mutable, which means that we can change their content.

Please check: Copy an object

We have a list of numbers, and we want to group them by the number of digits:

numbers = [1,256,64,65536,186000,1024,8,16,1905]
d = {}
for n in numbers:
    d.setdefault(len(str(n)),[]).append(n)
print d

Or:

numbers = [1,256,64,65536,186000,1024,8,16,1905]

d = {len(str(i)):[] for i in numbers}
print(f"group_by_digit = {d}")

for x in numbers:
    d[len(str(x))].append(x)
print(f"group_by_digit = {d}")

Output:

{1: [1, 8], 2: [64, 16], 3: [256], 4: [1024, 1905], 5: [65536], 6: [186000]}

Write a program which will find all such numbers which are divisible by 7 but are not a multiple of 3, between 1 and 100 (both included). The numbers obtained should be printed in a colon(:)-separated sequence on a single line as shown below:

7:14:28:35:49:56:70:77:91:98

The code should look like this:

l = []
for n in range(1,101):
    if n % 7 == 0 and n % 3 != 0:
        l.append(str(n))
print ':'.join(l)

We want to convert binary numbers to integers:

"0100,0011,1010,1001" => [4, 3, 10, 9]

Here is the code:

s1 = "0100,0011,1010,1001"
s2 = s1.split(',')
num = []
for s in s2:
    n = 0
    for i,v in enumerate(s[::-1]): # reverse 0100->0010
        n += (2**int(i)) * int(v)
    num.append(n)
print num

We want to write a code that computes the value of a+aa+aaa+aaaa with a given `digit` as the value of `a`. Suppose the following input is supplied to the program:

N = 9
DIGIT = 4

Then, the output should be:

9+99+999+9999 = 11106

Here is the code:

N = 9
DIGIT = 4
s = 0
for i in range(1,DIGIT+1):
    s += int(str(N)*i) # '9'*3 = '999'
print s

We have a file that has deposit(D) and withdrawal(W) records:

D 500
D 300
W 100
D 400
W 100

We want to calculate the balance after reading the bank records, and it should be 1,000. The code looks like this:

f = open("bank.txt")
balance = 0
for line in f:
    item = line.rstrip().split()
    if item[0] == 'D':
        balance += int(item[1])
    else:
        balance -= int(item[1])
print balance
f.close()

We can take only the words composed of digits from a given string using re.findall():

s = """Solar system:
Planets 8
Dwarf Planets: 5
Moons: Known = 149 | Provisional = 24 | Total = 173
Comets: More than 3400
Asteroids: More than 715000"""

import re
d = re.findall('\d+',s)
print d

Output:

['8', '5', '149', '24', '173', '3400', '715000']

We have a count of 35 heads and 94 legs among the chickens and pigs in a farm. How many pigs and how many chickens do we have?

# 35 heads and 94 legs among the chickens and pigs
# c + p = 35
# 2*c + 4*p = 94
heads = 35
legs = 94
for c in range(heads):
    p = 35-c
    if 2*c + 4*p == 94:
            print c,p

The answer is:

23 12

Find the highest possible product that we can get by multiplying any 3 numbers from an input array:

def mx3(a):
    b = sorted(a)

    m0 = b[0]
    m1 = b[1]
    x1 = b[-1]
    x2 = b[-2]
    x3 = b[-3]

    if m0 < 0 and m1 < 0:
        return max(m0*m1*x1,x1*x2*x3)

    else:
        return x1*x2*x3

l1 = [10,20,5,2,7,9,3,4]   
l2 = [10,-20,5,2,7,9,3,4]
l3 = [-10,-20,5,2,7,9,-3,4]
l4 = [10,-20,5,2,7,9,-3,4]

print mx3(l1)
print mx3(l2)
print mx3(l3)
print mx3(l4)

Output:

1800
630
1800
630

Implement a queue with a size limit of 3.

class Queue:
    def __init__(self):
        self.items = []
    
    def put(self, k,v):
        if len(self.items) >= 3:
            self.items.pop(0)
        self.items.append((k,v))
    
    def get(self,k):
        for x in self.items:
            if k == x[0]:
                return x[1]
        return -1
    
    def __repr__(self):
        return str(self.items)
        
q = Queue()
q.put(1,111), print(q)
q.put(2,222), print(q)
q.put(3,333), print(q)
q.put(4,444), print(q)
print(q.get(4))
print(q.get(10))  

Output:

[(1, 111)]
[(1, 111), (2, 222)]
[(1, 111), (2, 222), (3, 333)]
[(2, 222), (3, 333), (4, 444)]
444
-1

1. Python Coding Questions I
2. Python Coding Questions II
3. Python Coding Questions III
4. Python Coding Questions IV
5. Python Coding Questions V
6. Python Coding Questions VI
7. Python Coding Questions VII
8. Python Coding Questions VIII
9. Python Coding Questions IX
10. Python Coding Questions X

List of codes Q & A

1. Merging two sorted list
2. Get word frequency - initializing dictionary
3. Initializing dictionary with list
4. map, filter, and reduce
5. Write a function f() - yield
6. What is __init__.py?
7. Build a string with the numbers from 0 to 100, "0123456789101112..."
8. Basic file processing: Printing contents of a file - "with open"
9. How can we get home directory using '~' in Python?
10. The usage of os.path.dirname() & os.path.basename() - os.path
11. Default Libraries
12. range vs xrange
13. Iterators
14. Generators
15. Manipulating functions as first-class objects
16. docstrings vs comments
17. using lambdda
18. classmethod vs staticmethod
19. Making a list with unique element from a list with duplicate elements
20. What is map?
21. What is filter and reduce?
22. *args and **kwargs
23. mutable vs immutable
24. Difference between remove, del and pop on lists
25. Join with new line
26. Hamming distance
27. Floor operation on integers
28. Fetching every other item in the list
29. Python type() - function
30. Dictionary Comprehension
31. Sum
32. Truncating division
33. Python 2 vs Python 3
34. len(set)
35. Print a list of file in a directory
36. Count occurrence of a character in a Python string
37. Make a prime number list from (1,100)
38. Reversing a string - Recursive
39. Reversing a string - Iterative
40. Reverse a number
41. Output?
42. Merging overlapped range
43. Conditional expressions (ternary operator)
44. Packing Unpacking
45. Function args
46. Unpacking args
47. Finding the 1st revision with a bug
48. Which one has higher precedence in Python? - NOT, AND , OR
49. Decorator(@) - with dollar sign($)
50. Multi-line coding
51. Recursive binary search
52. Iterative binary search
53. Pass by reference
54. Simple calculator
55. iterator class that returns network interfaces
56. Converting domain to ip
57. How to count the number of instances
58. Python profilers - cProfile
59. Calling a base class method from a child class that overrides it
60. How do we find the current module name?
61. Why did changing list 'newL' also change list 'L'?
62. Constructing dictionary - {key:[]}
63. Colon separated sequence
64. Converting binary to integer
65. 9+99+999+9999+...
66. Calculating balance
67. Regular expression - findall
68. Chickens and pigs
69. Highest possible product
70. Implement a queue with a limited size
71. Copy an object
72. Filter
73. Products
74. Pickle
75. Overlapped Rectangles
76. __dict__
77. Fibonacci I - iterative, recursive, and via generator
78. Fibonacci II - which method?
79. Fibonacci III - find last two digits of Nth Fibonacci number
80. Write a Stack class returning Max item at const time A
81. Write a Stack class returning Max item at const time B
82. Finding duplicate integers from a list - 1
83. Finding duplicate integers from a list - 2
84. Finding duplicate integers from a list - 3
85. Reversing words 1
86. Parenthesis, a lot of them
87. Palindrome / Permutations
88. Constructing new string after removing white spaces
89. Removing duplicate list items
90. Dictionary exercise
91. printing numbers in Z-shape
92. Factorial
93. lambda
94. lambda with map/filter/reduce
95. Number of integer pairs whose difference is K
96. iterator vs generator
97. Recursive printing files in a given directory
98. Bubble sort
99. What is GIL (Global Interpreter Lock)?
100. Word count using collections
101. Pig Latin
102. List of anagrams from a list of words
103. lamda with map, filer and reduce functions
104. Write a code sending an email using gmail
105. histogram 1 : the frequency of characters
106. histogram 2 : the frequency of ip-address
107. Creating a dictionary using tuples
108. Getting the index from a list
109. Looping through two lists side by side
110. Dictionary sort with two keys : primary / secondary keys
111. Writing a file downloaded from the web
112. Sorting csv data
113. Reading json file
114. Sorting class objects
115. Parsing Brackets
116. Printing full path
117. str() vs repr()
118. Missing integer from a sequence
119. Polymorphism
120. Product of every integer except the integer at that index
121. What are accessors, mutators, and @property?
122. N-th to last element in a linked list
123. Implementing linked list
124. Removing duplicate element from a list
125. List comprehension
126. .py vs .pyc
127. Binary Tree
128. Print 'c' N-times without a loop
129. Quicksort
130. Dictionary of list
131. Creating r x c matrix
132. Transpose of a matrix
133. str.isalpha() & str.isdigit()
134. Regular expression
135. What is Hashable? Immutable?
136. Convert a list to a string
137. Convert a list to a dictionary
138. List - append vs extend vs concatenate
139. Use sorted(list) to keep the original list
140. list.count()
141. zip(list,list) - join elements of two lists
142. zip(list,list) - weighted average with two lists
143. Intersection of two lists
144. Dictionary sort by value
145. Counting the number of characters of a file as One-Liner
146. Find Armstrong numbers from 100-999
147. Find GCF (Greatest common divisor)
148. Find LCM (Least common multiple)
149. Draws 5 cards from a shuffled deck
150. Dictionary order by value or by key
151. Regular expression - re.split()
152. Regular expression : re.match() vs. re.search()
153. Regular expression : re.match() - password check
154. Regular expression : re.search() - group capturing
155. Regular expression : re.findall() - group capturin
156. Prime factors : n = products of prime numbers
157. Valid IPv4 address
158. Sum of strings
159. List rotation - left/right
160. shallow/deep copy
161. Converting integer to binary number
162. Creating a directory and a file
163. Creating a file if not exists
164. Invoking a python file from another
165. Sorting IP addresses
166. Word Frequency
167. Printing spiral pattern from a 2D array - I. Clock-wise
168. Printing spiral pattern from a 2D array - II. Counter-Clock-wise
169. Find a minimum integer not in the input list
170. I. Find longest sequence of zeros in binary representation of an integer
171. II. Find longest sequence of zeros in binary representation of an integer - should be surrounded with 1
172. Find a missing element from a list of integers
173. Find an unpaired element from a list of integers
174. Prefix sum : Passing cars
175. Prefix sum : count the number of integers divisible by k in range [A,B]
176. Can make a triangle?
177. Dominant element of a list
178. Minimum perimeter
179. MinAbsSumOfTwo
180. Ceiling - Jump Frog
181. Brackets - Nested parentheses
182. Brackets - Nested parentheses of multiple types
183. Left rotation - list shift
184. MaxProfit
185. Stack - Fish
186. Stack - Stonewall
187. Factors or Divisors
188. String replace in files 1
189. String replace in files 2
190. Using list as the default_factory for defaultdict
191. Leap year
192. Capitalize
193. Log Parsing
194. Getting status_code for a site
195. 2D-Array - Max hourglass sum
196. New Year Chaos - list
197. List (array) manipulation - list
198. Hash Tables: Ransom Note
199. Count Triplets with geometric progression
200. Strings: Check if two strings are anagrams
201. Strings: Making Anagrams
202. Strings: Alternating Characters
203. Special (substring) Palindrome
204. String with the same frequency of characters
205. Common Child
206. Fraudulent Activity Notifications
207. Maximum number of toys
208. Min Max Riddle
209. Poisonous Plants with Pesticides
210. Common elements of 2 lists - Complexity
211. Get execution time using decorator(@)
212. Conver a string to lower case and split using decorator(@)
213. Python assignment and memory location
214. shallow copy vs deep copy for compound objects (such as a list)
215. Generator with Fibonacci
216. Iterator with list
217. Second smallest element of a list
218. *args, **kargs, and positional args
219. Write a function, fn('x','y',3) that returns ['x1', 'y1', 'x2', 'y2', 'x3', 'y3']
220. sublist or not
221. any(), all()
222. Flattening a list
223. Select an element from a list
224. Circularly identical lists
225. Difference between two lists
226. Reverse a list
227. Split a list with a step
228. Break a list and make chunks of size n
229. Remove duplicate consecutive elements from a list
230. Combination of elements from two lists
231. Adding a sublist
232. Replace the first occurence of a value
233. Sort the values of the first list using the second list
234. Transpose of a matrix (nested list)
235. Binary Gap
236. Powerset
237. Round Robin
238. Fixed-length chunks or blocks
239. Accumulate
240. Dropwhile
241. Groupby
242. Simple product
243. Simple permutation
244. starmap(fn, iterable)
245. zip_longest(*iterables, fillvalue=None)
246. What is the correct way to write a doctest?
247. enumerate(iterable, start=0)
248. collections.defaultdict - grouping a sequence of key-value pairs into a dictionary of lists
249. What is the purpose of the 'self' keyword when defining or calling instance methods?
250. collections.namedtuple(typename, field_names, *, rename=False, defaults=None, module=None)
251. zipped
252. What is key difference between a set and a list?
253. What does a class's init() method do?
254. Class methods
255. Permutations and combinations of ['A','B','C']
256. Sort list of dictionaries by values
257. Return a list of unique words
258. hashlib
259. encode('utf-8')
260. Reading in CSV file
261. Count capital letters in a file
262. is vs ==
263. Create a matrix : [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
264. Binary to integer and check if it's the power of 2
265. urllib.request.urlopen() and requests
266. Game statistics
267. Chess - pawn race
268. Decoding a string
269. Determinant of a matrix - using numpy.linalg.det()
270. Revenue from shoe sales - using collections.Counter()
271. Rangoli
272. Unique characters
273. Valid UID
274. Permutations of a string in lexicographic sorted order
275. Nested list
276. Consecutive digit count
277. Find a number that occurs only once
278. Sorting a two-dimensional array
279. Reverse a string
280. Generate random odd numbers in a range
281. Shallow vs Deep copy
282. Transpose matrix
283. Are Arguments in Python Passed by Value or by Reference?
284. re: Is a string alphanumeric?
285. reversed()
286. Caesar's cipher, or shift cipher, Caesar's code, or Caesar shift
287. Every other words
288. re: How can we check if an email address is valid or not?
289. re: How to capture temperatures of a text
290. re.split(): How to split a text.
291. How can we merge two dictionaries?
292. How can we combine two dictionaries?
293. What is the difference between a generator and a list?
294. Pairs of a given array A whose sum value is equal to a target value N
295. Adding two integers without plus
296. isinstance() vs type()
297. What is a decorator?
298. In Python slicing, what does my_list[-3:2:-2] slice do?
299. Revisit sorting dict - counting chars in a text file
300. re: Transforming a date format using re.sub
301. How to replace the newlines in csv file with tabs?
302. pandas.merge
303. How to remove duplicate charaters from a string?
304. Implement a class called ComplexNumber
305. Find a word frequency
306. Get the top 3 most frequent characters of a string
307. Just seen and ever seen
308. Capitalizing the full name
309. Counting Consequitive Characters
310. Calculate Product of a List of Integers Provided using input()
311. How many times a substring appears in a string
312. Hello, first_name last_name
313. String validators
314. Finding indices that a char occurs in a list
315. itertools combinations