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Exercises


Strings and user input

  • Read about the Bytes literal from docs.python: String and Bytes literals. See also stackoverflow: What is the difference between a string and a byte string?
  • If you check out docs.python: int() function, you'll see that the int() function accepts an optional argument. Write a program that asks the user for hexadecimal number as input. Then, use the int() function to convert the input string to an integer (you'll need the second argument for this). Add 5 and display the result in hexadecimal format.
  • Write a program to accept two input values. First can be either a number or a string value. Second is an integer value, which should be used to display the first value in centered alignment. You can use any character you prefer to surround the value, other than the default space character.
  • What happens if you use a combination of r, f and other such valid prefix characters while declaring a string literal? For example, rf'a\{5/2}'. What happens if you use the raw strings syntax and provide only a single \ character? Does the documentation describe these cases?
  • Try out at least two format specifiers not discussed in this chapter.
  • Given a = 5, display '{5}' as the output using f-strings.

Defining functions

  • Call the function before declaration and see what happens for this program:

    def greeting():
    print('-----------------------------')
    print('         Hello World         ')
    print('-----------------------------')

greeting()

  • For the program shown below, try these experiments:

    • add print statements for ip, op_length and styled_line variables at the end of the program (after the function calls)
    • pass a numeric value to the greeting() function
    • don't pass any argument while calling the greeting() function
    def greeting(ip):
    op_length = 10 + len(ip)
    styled_line = '-' * op_length
    print(styled_line)
    print(f'{ip:^{op_length}}')
    print(styled_line)

greeting('hi')
weather = 'Today would be a nice, sunny day'
greeting(weather)

  • Modify the script shown below as per these requirements:

    • what do you think will happen if you call the function as greeting(spacing=5, ip='Oh!')
    • make the spacing work for multicharacter style argument
    • accept another argument with a default value of single space character that determines the character to be used around the centered ip value
    def greeting(ip, style='-', spacing=10):
    op_length = spacing + len(ip)
    styled_line = style * op_length
    print(styled_line)
    print(f'{ip:^{op_length}}')
    print(styled_line)

greeting('hi')
greeting('bye', spacing=5)
greeting('hello', style='=')
greeting('good day', ':', 2)

Control structures

  • What would happen if you use < or <= or > or >= operators between numeric and string values?

  • Simplify the function shown below to a single statement (ternary operator won't be needed):

    def isodd(n):
    if n % 2:
        return True
    else:
        return False

  • Create this arithmetic progression -2, 1, 4, 7, 10, 13 using the range() function.

  • What value would you get during each iteration of for c in 'hello'?

  • Rewrite the below program using a for loop. Can you think of a scenario where you must use a while loop instead of for?

    count = int(input('Enter a positive integer: '))
while count > 0:
    print(count)
    count -= 1

print('Go!')

  • Use appropriate range() logic so that the if statement is no longer needed for the snippet shown below.

    for num in range(10):
    if num % 3:
        continue
    print(f'{num} * 2 = {num * 2}')

  • If you don't already know about FizzBuzz, check out the problem statement on rosettacode and implement it in Python. See also Why Can't Programmers.. Program?

  • Print all numbers from 1 to 1000 (inclusive) which reads the same in reversed form in both the binary and decimal formats. For example, 33 in decimal is 100001 in binary and both of these are palindromic. You can either implement your own logic or search online for palindrome testing in Python.

  • Write a function that returns the maximum nested depth of curly braces for a given string input. For example, '{{a+2}*{{b+{c*d}}+e*d}}' should give 4. Unbalanced or wrongly ordered braces like '{a}*b{' and '}a+b{' should return -1.


Importing and creating modules

  • For the program shown below:

    • add docstrings and check the output of the help() function using num_funcs, num_funcs.fact, etc as arguments.
    • check what would be the output of num_funcs.fact() for negative integers and floating-point numbers. Then import the math built-in module and repeat the process with math.factorial().
    def sqr(n):
    return n * n

def fact(n):
    total = 1
    for i in range(2, n+1):
        total *= i
    return total

num = 5
print(f'square of {num} is {sqr(num)}')
print(f'factorial of {num} is {fact(num)}')

Exception handling

  • Identify the syntax errors in the following code snippets. Try to spot them manually.

    # snippet 1:
def greeting()
    print('hello')

# snippet 2:
num = 5
if num = 4:
    print('what is going on?!')

# snippet 3:
greeting = “hi”

  • For the function shown below, add exception handling to gracefully process negative integers and floating-point numbers.

    def fact(n):
    total = 1
    for i in range(2, n+1):
        total *= i
    return total

  • Write a function num(ip) that accepts a single argument and returns the corresponding integer or floating-point number contained in the argument. Only int, float and str should be accepted as a valid input data type. Provide custom error messages if the input cannot be converted to a valid number. Examples are shown below:

    # int example
>>> num(0x1f)
31
# float example
>>> num(3.32)
3.32
# str examples
>>> num(' \t 52 \t')
52
>>> num('3.982e5')
398200.0

# wrong data type
>>> num(['1', '2.3'])
TypeError: not a valid input data type

# string input that cannot be converted to a valid int/float number
>>> num('foo')
ValueError: could not convert string to int or float

Debugging

  • Create an empty file named as math.py. In the same directory, create another program file that imports the math module and then uses some feature, print(math.pi) for example. What happens if you execute this program?

Testing

  • Randomly change the logic of the max_nested_braces() function shown below and see if any of the tests fail.

    def max_nested_braces(expr):
    max_count = count = 0
    for char in expr:
        if char == '{':
            count += 1
            if count > max_count:
                max_count = count
        elif char == '}':
            if count == 0:
                return -1
            count -= 1

    if count != 0:
        return -1
    return max_count

def test_cases():
    assert max_nested_braces('a*b') == 0
    assert max_nested_braces('a*b+{}') == 1
    assert max_nested_braces('a*{b+c}') == 1
    assert max_nested_braces('{a+2}*{b+c}') == 1
    assert max_nested_braces('a*{b+c*{e*3.14}}') == 2
    assert max_nested_braces('{{a+2}*{b+c}+e}') == 2
    assert max_nested_braces('{{a+2}*{b+{c*d}}+e}') == 3
    assert max_nested_braces('{{a+2}*{{b+{c*d}}+e*d}}') == 4
    assert max_nested_braces('a*b{') == -1
    assert max_nested_braces('a*{b+c}}') == -1
    assert max_nested_braces('}a+b{') == -1
    assert max_nested_braces('a*{b+c*{e*3.14}}}') == -1
    assert max_nested_braces('{{a+2}*{{b}+{c*d}}+e*d}}') == -1

if __name__ == '__main__':
    test_cases()
    print('all tests passed')

Tuple and Sequence operations

  • Given chars = tuple('hello'), what'd be the output of the expression chars[0] and the statement chars[0] = 'H'?

  • Given primes = (2, 3, 5, 7, 11):

    • what happens if you use primes[5] or primes[-6]?
    • what happens if you use primes[:5] or primes[-6:]?
    • is it possible to get the same output as primes[::-1] by using an explicit number for the stop value? If not, why not?

  • What do you think will happen for these cases, given nums = (1, 2):

    • a, b, c = nums
    • a, *b, c = nums
    • *a, *b = nums

  • Instead of the function shown below, write a custom logic that iterates only once over the input sequence and calculates both the minimum and maximum values simultaneously.

    def min_max(iterable):
    return min(iterable), max(iterable)

  • Change the below snippet such that the index starts from 1 instead of 0.

    nums = (42, 3.14, -2)
for idx, val in enumerate(nums):
    print(f'{idx}: {val:>5}')

  • For the program shown below:

    • add a default valued argument initial which should be used to initialize total instead of 0 for the sum_nums() function. For example, sum_nums(3, -8) should give -5 and sum_nums(1, 2, 3, 4, 5, initial=5) should give 20.
    • what would happen if you use sum_nums(initial=5, 2) to call this function?
    • what would happen if you have nums = (1, 2) and use sum_nums(*nums, initial=3) to call the function?
    • in what ways does this function differ from the sum() built-in function?
    def sum_nums(*args):
    total = 0
    for n in args:
        total += n
    return total

  • Write a function inner_product(seq1, seq2) that returns the sum of product of corresponding elements of the two sequences. For example, the result should be 44 for (1, 3, 5) and (2, 4, 6) passed as arguments to this function.


List

  • What happens if you pass an iterable to the append() method and a non-iterable value to the extend() method on a list object? Also, what happens if you pass multiple values to both these methods?

  • Check what happens if you pass a list value to the insert() method. Also, what happens if you pass more than one object?

  • Read docs.python HOWTOs: Sorting and implement the below examples using the operator module instead of lambda expressions.

    # based on the second element of each item
>>> items = [('bus', 10), ('car', 20), ('jeep', 3), ('cycle', 5)]
>>> sorted(items, key=lambda e: e[1], reverse=True)
[('car', 20), ('bus', 10), ('cycle', 5), ('jeep', 3)]

# based on the number of words, assuming space as the word separator
>>> dishes = ('Poha', 'Aloo tikki', 'Baati', 'Khichdi', 'Makki roti')
>>> sorted(dishes, key=lambda s: s.count(' '), reverse=True)
['Aloo tikki', 'Makki roti', 'Poha', 'Baati', 'Khichdi']

  • Given nums = [1, 4, 5, 2, 51, 3, 6, 22], determine and implement the sorting condition based on the required output shown below:

    • [4, 2, 6, 22, 1, 5, 51, 3]
    • [2, 4, 6, 22, 1, 3, 5, 51]
    • [22, 6, 4, 2, 51, 5, 3, 1]

  • For the random.sample() method, see what happens if you pass a slice size greater than the number of elements present in the input sequence.

  • Write a function that returns the product of a sequence of numbers. Empty sequence or sequence containing non-numerical values should raise TypeError.

    • product([-4, 2.3e12, 77.23, 982, 0b101]) should give -3.48863356e+18
    • product(range(2, 6)) should give 120
    • product(()) and product(['a', 'b']) should raise TypeError

  • Write a function that removes dunder names from the dir() output.

    >>> remove_dunder(list)
['append', 'clear', 'copy', 'count', 'extend', 'index',
 'insert', 'pop', 'remove', 'reverse', 'sort']
>>> remove_dunder(tuple)
['count', 'index']

Mutability

  • Use the id() function to verify that the identity of the last two elements of the nums_2d variable in the below example is the same as the identity of both the elements in the last_two variable.

    nums_2d = ([1, 3, 2, 10], [1.2, -0.2, 0, 2], [100, 200])
last_two = nums_2d[-2:]

  • Create a deepcopy of only the first two elements of the nums_2d object shown below.

    nums_2d = [[1, 3, 2, 10], [1.2, -0.2, 0, 2], [100, 200]]

Dict

  • Given fruits = dict(banana=12, papaya=5, mango=10, fig=100), what do you think will happen when you use a, *b, c = fruits?
  • Given nums = [1, 4, 6, 22, 3, 5, 4, 3, 6, 2, 1, 51, 3, 1], keep only the first occurrences of a value from this list without changing the order of elements. The output should be [1, 4, 6, 22, 3, 5, 2, 51].

Set

  • Write a function that checks whether an iterable has duplicate values or not.

    >>> has_duplicates('pip')
True
>>> has_duplicates((3, 2))
False

  • What does your function return for has_duplicates([3, 2, 3.0])?


Text processing

  • Check what happens if you pass multiple string values separated by comma to the join() string method instead of an iterable.

  • Read the documentation for:

    • str methods translate() and maketrans()
    • built-in function ord()
    • string module

  • From str documentation, go through all the methods that start with is.

  • Read the documentation for the str methods rsplit(), partition() and rpartition()

  • Write a function that checks if two strings are anagrams irrespective of case (assume that the input is made up of alphabets only).

    >>> anagram('god', 'Dog')
True
>>> anagram('beat', 'table')
False
>>> anagram('Beat', 'abet')
True

  • Read the documentation and implement these formatting examples with the equivalent str methods.

    >>> fruit = 'apple'

>>> f'{fruit:=>10}'
'=====apple'
>>> f'{fruit:=<10}'
'apple====='
>>> f'{fruit:=^10}'
'==apple==='

>>> f'{fruit:^10}'
'  apple   '

  • Write a function that returns a list of words present in the input string.

    >>> words('"Hi", there! How *are* you? All fine here.')
['Hi', 'there', 'How', 'are', 'you', 'All', 'fine', 'here']
>>> words('This-Is-A:Colon:Separated,Phrase')
['This', 'Is', 'A', 'Colon', 'Separated', 'Phrase']

Comprehensions and Generator expressions

  • Write a function that returns a dictionary sorted by values in ascending order.

    >>> marks = dict(Rahul=86, Ravi=92, Rohit=75, Rajan=79, Ram=92)
>>> sort_by_value(marks)
{'Rohit': 75, 'Rajan': 79, 'Rahul': 86, 'Ravi': 92, 'Ram': 92}

  • Write a function that returns a list of string slices as per the following rules:

    • return the input string as the only element if its length is less than 3 characters
    • otherwise, return all slices that have 2 or more characters
    >>> word_slices('')
['']
>>> word_slices('i')
['i']
>>> word_slices('to')
['to']
>>> word_slices('table')
['ta', 'tab', 'tabl', 'table', 'ab', 'abl', 'able', 'bl', 'ble', 'le']

  • Square even numbers and cube odd numbers. For example, [321, 1, -4, 0, 5, 2] should give [33076161, 1, 16, 0, 125, 4] as the output.

  • Calculate sum of squares of the numbers, only if the square value is less than 50. Output for (7.1, 1, -4, 8, 5.1, 12) should be 43.01.


Dealing with files

  • Write a program that reads a known filename f1.txt which contains a single column of numbers. Your task is to display the sum of these numbers, which is 10485.14 for the given example.

    $ cat f1.txt 
8
53
3.14
84
73e2
100
2937

  • Read the documentation for glob.glob() and write a program to list all files ending with .txt in the current directory as well as sub-directories, recursively.


Executing external commands

  • Read the subprocess.run documentation and modify the below ls example to:

    • redirect the stderr stream to /dev/null
    • automatically raise an exception when the exit status is non-zero
    >>> import subprocess

>>> process = subprocess.run(('ls', 'xyz.txt'))
ls: cannot access 'xyz.txt': No such file or directory
>>> process.returncode
2

Command line arguments

  • Modify the sum_two_nums.py program to handle TypeError exceptions as well. Instead of the output shown below, inform the user about the error using sys.exit() method.

    # sum_two_nums.py
import ast
import sys

try:
    num1, num2 = sys.argv[1:]
    total = ast.literal_eval(num1) + ast.literal_eval(num2)
except ValueError:
    sys.exit('Error: Please provide exactly two numbers as arguments')
else:
    print(f'{num1} + {num2} = {total}')
    $ python3.13 sum_two_nums.py 2 [1]
Traceback (most recent call last):
  File "/home/learnbyexample/Python/programs/sum_two_nums.py", line 6, in <module>
    total = ast.literal_eval(num1) + ast.literal_eval(num2)
            ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
TypeError: unsupported operand type(s) for +: 'int' and 'list'

  • Write a program that accepts one or more numbers as CLI arguments. Calculate and display the following details about the input — sum, product and average.

  • Add an optional argument -o, --output to store the output in a file for the program shown below.

    import argparse, sys

parser = argparse.ArgumentParser()
parser.add_argument('file', nargs='?',
                    type=argparse.FileType('r'), default=sys.stdin,
                    help="input file to be sorted")
parser.add_argument('-u', '--unique', action='store_true',
                    help="sort uniquely")
args = parser.parse_args()

ip_lines = args.file.readlines()
if args.unique:
    ip_lines = set(ip_lines)

op_lines = sorted(ip_lines, key=lambda s: (s.rsplit('.', 1)[-1], s))
for line in op_lines:
    print(line, end='')

More exercises

If you'd like even more exercises to test your understanding, check out these excellent resources:

See also Python Programming Exercises, Gently Explained — a free ebook that includes gentle explanations of the problem, the prerequisite coding concepts you'll need to understand the solution, etc.