1. A function is a block of code that performs a specific task when the function is called (invoked). You can use functions to make your code reusable, better organized, and more readable. Functions can have parameters and return values.<\/p>\n
<\/p>\n
2. There are at least four basic types of functions in Python:<\/p>\n
print()<\/code> function). You can see a complete list of Python built-in functions at https:\/\/docs.python.org\/3\/library\/functions.html<\/em>.<\/li>\n- the ones that come from pre-installed modules<\/strong>.<\/li>\n
- user-defined functions<\/strong> which are written by users for users – you can write your own functions and use them freely in your code,<\/li>\n
- the
lambda<\/code> functions<\/li>\n<\/ul>\n3. You can define your own function using the def<\/code> keyword and the following syntax:<\/p>\ndef your_function(optional parameters):\r\n # the body of the function<\/pre>\nYou can define a function which doesn’t take any arguments, e.g.:<\/p>\n
def message(): # defining a function\r\n print(\"Hello\") # body of the function\r\n\r\nmessage() # calling the function<\/pre>\nYou can define a function which takes arguments, too, just like the one-parameter function below:<\/p>\n
def hello(name): # defining a function\r\n print(\"Hello,\", name) # body of the function\r\n\r\nname = input(\"Enter your name: \")\r\n\r\nhello(name) # calling the function\r\n<\/pre>\n <\/p>\n
4. You can pass information to functions by using parameters. Your functions can have as many parameters as you need.<\/p>\n
An example of a one-parameter function:<\/p>\n
def hi(name):\r\n print(\"Hi,\", name)\r\n\r\nhi(\"Greg\")<\/pre>\nAn example of a two-parameter function:<\/p>\n
def hi_all(name_1, name_2):\r\n print(\"Hi,\", name_2)\r\n print(\"Hi,\", name_1)\r\n\r\nhi_all(\"Sebastian\", \"Konrad\")<\/pre>\nAn example of a three-parameter function:<\/p>\n
def address(street, city, postal_code):\r\n print(\"Your address is:\", street, \"St.,\", city, postal_code)\r\n\r\ns = input(\"Street: \")\r\np_c = input(\"Postal Code: \")\r\nc = input(\"City: \")\r\n\r\naddress(s, c, p_c)<\/pre>\n5. You can pass arguments to a function using the following techniques:<\/p>\n
\n- positional argument passing in which the order of arguments passed matters (Ex. 1),<\/li>\n
- keyword (named) argument passing in which the order of arguments passed doesn’t matter (Ex. 2),<\/li>\n
- a mix of positional and keyword argument passing (Ex. 3).<\/li>\n<\/ul>\n
Ex. 1\r\ndef subtra(a, b):\r\nprint(a - b)\r\n\r\nsubtra(5, 2) # outputs: 3\r\nsubtra(2, 5) # outputs: -3\r\n\r\nEx. 2\r\ndef subtra(a, b):\r\nprint(a - b)\r\n\r\nsubtra(a=5, b=2) # outputs: 3\r\nsubtra(b=2, a=5) # outputs: 3\r\n\r\nEx. 3\r\ndef subtra(a, b):\r\nprint(a - b)\r\n\r\nsubtra(5, b=2) # outputs: 3\r\nsubtra(5, 2) # outputs: 3<\/pre>\nIt’s important to remember that you have to put positional arguments before keyword arguments<\/strong>. Positional arguments mustn’t follow keyword arguments. That’s why if you try to run the following snippet:<\/p>\ndef subtra(a, b):\r\nprint(a - b)\r\n\r\nsubtra(5, b=2) # outputs: 3\r\nsubtra(a=5, 2) # Syntax Error<\/pre>\nPython will not let you do it by signalling a SyntaxError.<\/p>\n
6. You can use the keyword argument passing technique to pre-define a value for a given argument:<\/p>\n
def name(first_name, last_name=\"Smith\"):\r\nprint(first_name, last_name)\r\n\r\nname(\"Andy\") # outputs: Andy Smith\r\nname(\"Betty\", \"Johnson\") # outputs: Betty Johnson (the keyword argument replaced by \"Johnson\")<\/pre>\n <\/p>\n
7. You can use the return<\/code> keyword to tell a function to return some value. The return statement exits the function, e.g.:<\/p>\ndef multiply(a, b):\r\n return a * b\r\nprint(multiply(3, 4)) # outputs: 12\r\n\r\ndef multiply(a, b):\r\n return\r\nprint(multiply(3, 4)) # outputs: None<\/pre>\n <\/p>\n
8. The result of a function can be easily assigned to a variable, e.g.:<\/p>\n
def wishes():\r\n return \"Happy Birthday!\"\r\n\r\nw = wishes()\r\n\r\nprint(w) # outputs: Happy Birthday!<\/pre>\n <\/p>\n
Look at the difference in output in the following two examples:<\/p>\n
# Example 1\r\ndef wishes():\r\n print(\"My Wishes\")\r\n return \"Happy Birthday\"\r\n\r\nwishes() # outputs: My Wishes\r\n\r\n# Example 2\r\ndef wishes():\r\n print(\"My Wishes\")\r\n return \"Happy Birthday\"\r\n\r\nprint(wishes())\r\n\r\n# outputs: My Wishes\r\n# Happy Birthday<\/pre>\n <\/p>\n
9. You can use a list as a function’s argument, e.g.:<\/p>\n
def hi_everybody(my_list):\r\n for name in my_list:\r\n print(\"Hi,\", name)\r\n\r\nhi_everybody([\"Adam\", \"John\", \"Lucy\"])<\/pre>\n <\/p>\n
10. A list can be a function result, too, e.g.:<\/p>\n
def create_list(n):\r\n my_list = []\r\n for i in range(n):\r\n my_list.append(i)\r\n return my_list\r\n\r\nprint(create_list(5))<\/pre>\n <\/p>\n
11. You can use the return<\/code> keyword to tell a function to return some value. The return<\/code> statement exits the function, e.g.:<\/code><\/p>\n\n\ndef multiply(a, b):\r\n return a * b\r\n\r\nprint(multiply(3, 4)) # outputs: 12\r\n\r\ndef multiply(a, b):\r\n return\r\n\r\nprint(multiply(3, 4)) # outputs: None<\/pre>\n <\/p>\n
12. The result of a function can be easily assigned to a variable, e.g.:<\/p>\n<\/div>\n<\/div>\n
def wishes(): \r\n return \"Happy Birthday!\" \r\n\r\nw = wishes() \r\nprint(w) # outputs: Happy Birthday!<\/pre>\nLook at the difference in output in the following two examples:<\/code><\/p>\n\n\n# Example 1\r\ndef wishes():\r\n print(\"My Wishes\")\r\n return \"Happy Birthday\"\r\n\r\nwishes() # outputs: My Wishes\r\n\r\n# Example 2\r\ndef wishes():\r\n print(\"My Wishes\")\r\n return \"Happy Birthday\"\r\n\r\nprint(wishes())\r\n\r\n# outputs: My Wishes\r\n# Happy Birthday<\/pre>\n <\/p>\n
13. You can use a list as a function’s argument, e.g.:<\/p>\n<\/div>\n<\/div>\n
\n\ndef hi_everybody(my_list):\r\n for name in my_list:\r\n print(\"Hi,\", name)\r\n\r\nhi_everybody([\"Adam\", \"John\", \"Lucy\"])<\/pre>\n <\/p>\n
14. A list can be a function result, too, e.g.:<\/p>\n<\/div>\n<\/div>\n
\n\ndef create_list(n):\r\n my_list = []\r\n for i in range(n):\r\n my_list.append(i)\r\n return my_list\r\n\r\nprint(create_list(5))<\/pre>\n<\/div>\n<\/div>\n <\/p>\n
15. A variable that exists outside a function has a scope inside the function body (Example 1) unless the function defines a variable of the same name (Example 2, and Example 3), e.g.:<\/p>\n
Example 1:<\/p>\n
var = 2 \r\ndef mult_by_var(x): \r\n return x * var \r\n\r\nprint(mult_by_var(7)) # outputs: 14<\/pre>\nExample 2:<\/p>\n
def mult(x): \r\n var = 5 \r\n return x * var \r\n\r\nprint(mult(7)) # outputs: 35<\/pre>\n <\/p>\n
Example 3:<\/p>\n
def mult(x): \r\n var = 7 \r\n return x * var\r\n\r\nvar = 3 \r\nprint(mult(7)) # outputs: 49<\/pre>\n <\/p>\n
16. A variable that exists inside a function has a scope inside the function body (Example 4), e.g.:<\/p>\n
Example 4:<\/p>\n
def adding(x): \r\n var = 7 \r\n return x + var \r\n\r\nprint(adding(4)) # outputs: 11 \r\nprint(var) # NameError<\/pre>\n <\/p>\n
17. You can use the global<\/code> keyword followed by a variable name to make the variable’s scope global, e.g.:<\/p>\n\n\nvar = 2\r\nprint(var) # outputs: 2\r\n\r\ndef return_var():\r\n global var\r\n var = 5\r\n return var\r\n\r\nprint(return_var()) # outputs: 5\r\nprint(var) # outputs: 5<\/pre>\n <\/p>\n
18. Is it worth checking how it works with lists (do you recall the peculiarities of assigning list slices versus assigning lists as a whole?).<\/div>\n<\/div>\n<\/div>\nThe following example will shed some light on the issue:<\/p>\n
<\/code><\/p>\n\n\ndef my_function(my_list_1):\r\n print(\"Print #1:\", my_list_1)\r\n print(\"Print #2:\", my_list_2)\r\n my_list_1 = [0, 1]\r\n print(\"Print #3:\", my_list_1)\r\n print(\"Print #4:\", my_list_2)\r\n\r\nmy_list_2 = [2, 3]\r\nmy_function(my_list_2)\r\nprint(\"Print #5:\", my_list_2)<\/pre>\nThe code’s output is:<\/p>\n<\/div>\n<\/div>\n
Print #1: [2, 3]\r\nPrint #2: [2, 3]\r\nPrint #3: [0, 1]\r\nPrint #4: [2, 3]\r\nPrint #5: [2, 3]<\/pre>\nIt seems that the former rule still works.<\/p>\n
Finally, can you see the difference in the example below:<\/code><\/p>\n\n\ndef my_function(my_list_1):\r\n print(\"Print #1:\", my_list_1)\r\n print(\"Print #2:\", my_list_2)\r\n del my_list_1[0] # Pay attention to this line.\r\n print(\"Print #3:\", my_list_1)\r\n print(\"Print #4:\", my_list_2)\r\n\r\nmy_list_2 = [2, 3]\r\nmy_function(my_list_2)\r\nprint(\"Print #5:\", my_list_2)<\/pre>\nWe don’t change the value of the parameter my_list_1<\/code> (we already know it will not affect the argument), but instead modify the list identified by it.<\/p>\n<\/div>\n<\/div>\nThe output may be surprising. Run the code and check:<\/p>\n
Print #1: [2, 3] \r\nPrint #2: [2, 3] \r\nPrint #3: [3] \r\nPrint #4: [3] \r\nPrint #5: [3]<\/pre>\nLet’s explain it :<\/p>\n
\n- if the argument is a list, then changing the value of the corresponding parameter doesn’t affect the list (remember: variables containing lists are stored in a different way than scalars),<\/li>\n
- but if you change a list identified by the parameter (note: the list, not the parameter!), the list will reflect the change.<\/li>\n<\/ul>\n
<\/p>\n
19. A function can call other functions or even itself. When a function calls itself, this situation is known as recursion<\/strong>, and the function which calls itself and contains a specified termination condition (i.e., the base case – a condition which doesn’t tell the function to make any further calls to that function) is called a recursive<\/strong> function.<\/p>\n20. You can use recursive functions in Python to write clean, elegant code, and divide it into smaller, organized chunks<\/strong>. On the other hand, you need to be very careful as it might be easy to make a mistake and create a function which never terminates<\/strong>. You also need to remember that recursive calls consume a lot of memory<\/strong>, and therefore may sometimes be inefficient.<\/p>\nWhen using recursion, you need to take all its advantages and disadvantages into consideration.<\/p>\n
The factorial function is a classic example of how the concept of recursion can be put in practice:<\/p>\n
def factorial_function(n):\r\n if n < 0:\r\n return None\r\n if n < 2:\r\n return 1\r\n return n * factorial_function(n - 1)\r\n\r\nprint(factorial(4)) # 4 * 3 * 2 * 1 = 2<\/pre>\nFactorial without recursion:<\/p>\n
def factorial_function(n):\r\n if n < 0:\r\n return None\r\n if n < 2:\r\n return 1\r\n \r\n product = 1\r\n for i in range(2, n + 1):\r\n product *= i\r\n return product\r\n\r\n\r\nfor n in range(1, 6): # testing\r\n print(n, factorial_function(n))\r\n\r\n<\/pre>\n <\/p>\n
<\/code><\/p>\n\n\nExercise 1<\/em><\/p>\n<\/div>\n<\/div>\nWhat is the output of the following snippet?<\/p>\n
def intro(a=\"James Bond\", b=\"Bond\"):\r\n print(\"My name is\", b + \".\", a + \".\")\r\n\r\nintro()<\/pre>\n# My name is Bond. James Bond.<\/p>\n
<\/p>\n
Exercise 2<\/em><\/p>\nWhat is the output of the following snippet?<\/p>\n
def intro(a=\"James Bond\", b=\"Bond\"):\r\n print(\"My name is\", b + \".\", a + \".\")\r\n\r\nintro(b=\"Sean Connery\")<\/pre>\n# My name is Sean Connery. James Bond.<\/p>\n
<\/p>\n
Exercise 3<\/em><\/p>\nWhat is the output of the following snippet?<\/p>\n
def intro(a, b=\"Bond\"):\r\n print(\"My name is\", b + \".\", a + \".\")\r\n\r\nintro(\"Susan\")<\/pre>\n# My name is Bond. Susan.<\/p>\n
<\/p>\n
Exercise 4<\/em><\/p>\nWhat is the output of the following snippet?<\/p>\n
def add_numbers(a, b=2, c):\r\n print(a + b + c)\r\n\r\nadd_numbers(a=1, c=3)\r\n\r\n<\/pre>\n# SyntaxError – a non-default argument (c) follows a default argument (b=2)<\/p>\n
A non-default arguments must be before a default arguments.<\/strong><\/p>\n <\/p>\n
Exercise 5<\/em><\/p>\nWhat is the output of the following snippet?<\/p>\n
def hi():\r\n return\r\n print(\"Hi!\")\r\n\r\nhi()<\/pre>\n# the function will return an implicit None value<\/p>\n
<\/p>\n
Exercise 6<\/em><\/p>\nWhat is the output of the following snippet?<\/p>\n
def is_int(data):\r\nif type(data) == int:\r\n return True\r\nelif type(data) == float:\r\n return False\r\n\r\nprint(is_int(5))\r\nprint(is_int(5.0))\r\nprint(is_int(\"5\"))\r\n<\/pre>\n <\/p>\n
True<\/p>\n
False<\/p>\n
None<\/p>\n
<\/p>\n
Exercise 7<\/em><\/p>\nWhat is the output of the following snippet?<\/p>\n
def even_num_lst(ran):\r\n lst = []\r\n for num in range(ran):\r\n if num % 2 == 0:\r\n lst.append(num)\r\n return lst\r\n\r\nprint(even_num_lst(11))\r\n<\/pre>\n <\/p>\n
[0, 2, 4, 6, 8, 10]<\/p>\n
<\/p>\n
Exercise 8<\/em><\/p>\nWhat is the output of the following snippet?<\/p>\n
def list_updater(lst):\r\n upd_list = []\r\n for elem in lst:\r\n elem **= 2\r\n upd_list.append(elem)\r\n return upd_list\r\n\r\nfoo = [1, 2, 3, 4, 5]\r\nprint(list_updater(foo))\r\n<\/pre>\n <\/p>\n
[1, 4, 9, 16, 25]<\/p>\n
<\/p>\n
Exercise 9<\/em>
\n<\/strong><\/p>\nWhat is the output of the following snippet?<\/code><\/p>\ndef hi(): \r\n return print(\"Hi!\") \r\n\r\nhi()<\/pre>\nthe function will return an implicit None<\/code> value<\/p>\n <\/p>\n
Exercise 10<\/em>
\n<\/strong><\/p>\nWhat is the output of the following snippet?<\/code><\/p>\ndef is_int(data): \r\n if type(data) == int: \r\n return True \r\n elif type(data) == float: \r\n return False \r\n\r\nprint(is_int(5)) \r\nprint(is_int(5.0)) \r\nprint(is_int(\"5\"))<\/code><\/pre>\n<\/code><\/p>\nTrue
\nFalse<\/code>
\nNone<\/code><\/p>\n <\/p>\n
Exercise 11<\/em>
\n<\/strong><\/p>\nWhat is the output of the following snippet?<\/p>\n
def even_num_lst(ran): \r\n lst = [] \r\n for num in range(ran): \r\n if num % 2 == 0: \r\n lst.append(num) \r\n return lst \r\n\r\nprint(even_num_lst(11))<\/pre>\n<\/div>\n[0, 2, 4, 6, 8, 10]<\/code><\/p>\n <\/p>\n
Exercise 12<\/em>
\n<\/strong><\/p>\nWhat is the output of the following snippet?<\/p>\n
def list_updater(lst): \r\n upd_list = [] \r\n for elem in lst: \r\n elem **= 2 upd_list.append(elem) \r\n return upd_list foo = [1, 2, 3, 4, 5] \r\n\r\nprint(list_updater(foo))<\/pre>\n<\/div>\n[1, 4, 9, 16, 25]<\/code><\/p>\n <\/p>\n
Exercise 13<\/em><\/p>\nWhat will happen when you try to run the following code?<\/p>\n
def message(): \r\n alt = 1 \r\n print(\"Hello, World!\") \r\n\r\nprint(alt)<\/pre>\n <\/p>\n
The NameError<\/code> exception will be thrown (NameError: name 'alt' is not defined<\/code>)<\/p>\n <\/p>\n
Exercise 14
\n<\/em><\/p>\nWhat is the output of the following snippet?<\/p>\n
a = 1 \r\ndef fun(): \r\n a = 2 \r\n print(a) \r\n\r\nfun() \r\nprint(a)<\/pre>\n2<\/p>\n
1<\/p>\n
<\/p>\n
Exercise 15<\/em>
\n<\/strong><\/p>\nWhat is the output of the following snippet?<\/p>\n
a = 1 \r\ndef fun(): \r\n global a \r\n a = 2 \r\n print(a) \r\n\r\nfun() \r\na = 3 \r\nprint(a)<\/pre>\n2<\/p>\n
3<\/p>\n
<\/p>\n
Exercise 16<\/em>
\n<\/strong><\/p>\nWhat is the output of the following snippet?<\/p>\n
a = 1 \r\ndef fun(): \r\n global a \r\n a = 2 \r\n print(a) \r\n\r\na = 3 \r\nfun() \r\nprint(a)<\/pre>\n2<\/p>\n
2<\/p>\n
<\/p>\n
Exercise 17<\/em><\/p>\nWhat will happen when you attempt to run the following snippet and why?<\/p>\n
def factorial(n): \r\n return n * factorial(n - 1) \r\n\r\nprint(factorial(4))<\/pre>\n <\/p>\n
The factorial function has no termination condition (no base case) so Python will raise an exception (RecursionError: maximum recursion depth exceeded<\/code>)<\/p>\n <\/p>\n
Exercise 18<\/em>
\n<\/strong><\/p>\nWhat is the output of the following snippet?<\/p>\n
def fun(a): \r\nif a > 30: \r\n return 3 \r\nelse: \r\n return a + fun(a + 3) \r\n\r\nprint(fun(25))<\/pre>\n<\/div>\n56<\/code><\/p>\n <\/p>\n
Exercise 19<\/em>
\n<\/strong><\/p>\nWhat is the output of the following snippet?<\/p>\n
def fun(in=2, out=3):\r\n return in*out\r\n\r\nprint (fun(3))<\/pre>\n <\/p>\n
invalid syntax<\/code><\/p>\nin<\/code> is a keyword, you can not use in<\/code> as a name of parametr<\/p>\n <\/p>\n
Ex. 20<\/p>\n
What is the output of the following code snippet?<\/p>\n
x = 10\r\n\r\ndef fun():\r\n global x\r\n if x >= 10:\r\n x += 100\r\n\r\nprint(x)<\/pre>\n <\/p>\n
10<\/code><\/p>\nThe code does not call the fun() function. As a result, the program does not execute the body of the function and the content of the variable x does not change. Therefore, the print statement returns 10 on the screen.<\/p>\n
<\/p>\n
Ex. 21<\/p>\n
\n\n\nWhat is the output of the following code snippet?<\/p>\n
\n\ndef fun(x, y = 1):\r\n return x * y\r\n\r\nprint(fun('3', 2))<\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n