Built in functions
In Python we have lots of built-in functions. Built-in functions are globally available for your use that mean you can make use of the built-in functions without importing or configuring. Some of the most commonly used Python built-in functions are the following: print(), len(), type(), int(), float(), str(), input(), list(), dict(), min(), max(), sum(), sorted(), open(), file(), help(), and dir(). In the following table you will see an exhaustive list of Python built-in functions taken from python documentation.
Let us open the Python shell and start using some of the most common built-in
functions.
Let us practice more by using different built-in functions
As you can see from the terminal above, Python has got reserved words. We do not use reserved words to declare variables or functions. We will cover variables in the next section.
I believe, by now you are familiar with built-in functions. Let us do one more practice of built-in functions and we will move on to the next section.
Variables
Variables store data in a computer memory. Mnemonic variables are recommended to use in many programming languages. A mnemonic variable is a variable name that can be easily remembered and associated. A variable refers to a memory address in which data is stored. Number at the beginning, special character, hyphen are not allowed when naming a variable. A variable can have a short name (like x, y, z), but a more descriptive name (firstname, lastname, age, country) is highly recommended.
Python Variable Name Rules A variable name must start with a letter or the underscore character A variable name cannot start with a number A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ ) Variable names are case-sensitive (firstname, Firstname, FirstName and FIRSTNAME) are different variables)
We will use standard Python variable naming style which has been adopted by many Python developers. Python developers use snake case(snake_case) variable naming convention. We use underscore character after each word for a variable containing more than one word(eg. first_name, last_name, engine_rotation_speed). The example below is an example of standard naming of variables, underscore is required when the variable name is more than one word.
When we assign a certain data type to a variable, it is called variable declaration. For instance in the example below my first name is assigned to a variable first_name. The equal sign is an assignment operator. Assigning means storing data in the variable. The equal sign in Python is not equality as in Mathematics.
Declaring Multiple Variable in a Line
Multiple variables can also be declared in one line:
Example: first_name, last_name, country, age, is_married = 'Asabeneh',
'Yetayeh', 'Helsink', 250, True print(first_name, last_name, country, age,
is_married) print('First name:', first_name) print('Last name: ', last_name)
print('Country: ', country) print('Age: ', age) print('Married: ', is_married)
Getting user input using the input() built-in function. Let us assign the data
we get from a user into first_name and age variables. Example: first_name =
input('What is your name: ') age = input('How old are you? ') print(first_name)
print(age) Data Types There are several data types in Python. To identify the
data type we use the type built-in function. I would like to ask you to focus on
understanding different data types very well. When it comes to programming, it
is all about data types. I introduced data types at the very beginning and it
comes again, because every topic is related to data types. We will cover data
types in more detail in their respective sections. Checking Data types and
Casting Check Data types: To check the data type of certain data/variable we use
the type Example: # Different python data types # Let's declare variables with
various data types first_name = 'Asabeneh' # str last_name = 'Yetayeh' # str
country = 'Finland' # str city= 'Helsinki' # str age = 250 # int, it is not my
real age, don't worry about it # Printing out types print(type('Asabeneh')) #
str print(type(first_name)) # str print(type(10)) # int print(type(3.14)) #
float print(type(1 + 1j)) # complex print(type(True)) # bool print(type([1, 2,
3, 4])) # list print(type({'name':'Asabeneh','age':250, 'is_married':250})) #
dict print(type((1,2))) # tuple print(type(zip([1,2],[3,4]))) # set Casting:
Converting one data type to another data type. We use int(), float(), str(),
list, set When we do arithmetic operations string numbers should be first
converted to int or float otherwise it will return an error. If we concatenate a
number with a string, the number should be first converted to a string. We will
talk about concatenation in String section. Example: # int to float num_int = 10
print('num_int',num_int) # 10 num_float = float(num_int) print('num_float:',
num_float) # 10.0 # float to int gravity = 9.81 print(int(gravity)) # 9 # int to
str num_int = 10 print(num_int) # 10 num_str = str(num_int) print(num_str) #
'10' # str to int or float num_str = '10.6' print('num_int', int(num_str)) # 10
print('num_float', float(num_str)) # 10.6 # str to list first_name = 'Asabeneh'
print(first_name) # 'Asabeneh' first_name_to_list = list(first_name)
print(first_name_to_list) # ['A', 's', 'a', 'b', 'e', 'n', 'e', 'h'] Numbers
Number data types in Python: Integers: Integer(negative, zero and positive)
numbers Example: ... -3, -2, -1, 0, 1, 2, 3 ... Floating Point Numbers(Decimal
numbers) Example: ... -3.5, -2.25, -1.0, 0.0, 1.1, 2.2, 3.5 ... Complex Numbers
Example: 1 + j, 2 + 4j, 1 - 1j Boolean A boolean data type represents one of the
two values: True or False. The use of these data types will be clear once we
start using the comparison operator. The first letter T for True and F for False
should be capital unlike JavaScript. Example: Boolean Values print(True)
print(False) Operators Python language supports several types of operators. In
this section, we will focus on few of them. • Assignment Operators Assignment
operators are used to assign values to variables. Let us take = as an example.
Equal sign in mathematics shows that two values are equal, however in Python it
means we are storing a value in a certain variable and we call it assignment or
a assigning value to a variable. The table below shows the different types of
python assignment operators, taken from w3school.
• Arithmetic Operators: 1. Addition(+): a + b 2. Subtraction(-): a – b 3.
Multiplication(*): a * b 4. Division(/): a / b 5. Modulus(%): a % b 6. Floor
division(//): a // b 7. Exponentiation(**): a ** b
Example:Integers # Arithmetic Operations in Python # Integers print('Addition:
', 1 + 2) # 3 print('Subtraction: ', 2 - 1) # 1 print('Multiplication: ', 2 * 3)
# 6 print ('Division: ', 4 / 2) # 2.0 Division in Python gives floating number
print('Division: ', 6 / 2) # 3.0 print('Division: ', 7 / 2) # 3.5
print('Division without the remainder: ', 7 // 2) # 3, gives without the
floating number or without the remaining print ('Division without the remainder:
',7 // 3) # 2 print('Modulus: ', 3 % 2) # 1, Gives the remainder
print('Exponentiation: ', 2 ** 3) # 9 it means 2 * 2 * 2 Example:Floats #
Floating numbers print('Floating Point Number, PI', 3.14) print('Floating Point
Number, gravity', 9.81) Example:Complex numbers # Complex numbers print('Complex
number: ', 1 + 1j) print('Multiplying complex numbers: ',(1 + 1j) * (1 - 1j))
Let's declare a variable and assign a number data type. I am going to use single
character variable but remember do not develop a habit of declaring such types
of variables. Variable names should be all the time mnemonic. Example: #
Declaring the variable at the top first a = 3 # a is a variable name and 3 is an
integer data type b = 2 # b is a variable name and 3 is an integer data type #
Arithmetic operations and assigning the result to a variable total = a + b diff
= a - b product = a * b division = a / b remainder = a % b floor_division = a //
b exponential = a ** b # I should have used sum instead of total but sum is a
built-in function - try to avoid overriding built-in functions print(total) # if
you do not label your print with some string, you never know where the result is
coming from print('a + b = ', total) print('a - b = ', diff) print('a * b = ',
product) print('a / b = ', division) print('a % b = ', remainder) print('a // b
= ', floor_division) print('a ** b = ', exponentiation) Example: print('==
Addition, Subtraction, Multiplication, Division, Modulus ==') # Declaring values
and organizing them together num_one = 3 num_two = 4 # Arithmetic operations
total = num_one + num_two diff = num_two - num_one product = num_one * num_two
div = num_two / num_one remainder = num_two % num_one # Printing values with
label print('total: ', total) print('difference: ', diff) print('product: ',
product) print('division: ', div) print('remainder: ', remainder) Let us start
start connecting the dots and start making use of what we already know to
calculate (area, volume,density, weight, perimeter, distance, force). Example: #
Calculating area of a circle radius = 10 # radius of a circle area_of_circle =
3.14 * radius ** 2 # two * sign means exponent or power print('Area of a
circle:', area_of_circle) # Calculating area of a rectangle length = 10 width =
20 area_of_rectangle = length * width print('Area of rectangle:',
area_of_rectangle) # Calculating a weight of an object mass = 75 gravity = 9.81
weight = mass * gravity print(weight, 'N') # Adding unit to the weight #
Calculate the density of a liquid mass = 75 # in Kg volume = 0.075 # in cubic
meter density = mass / volume # 1000 Kg/m^3 Comparison Operators In programming
we compare values, we use comparison operators to compare two values. We check
if a value is greater or less or equal to other value. The following table shows
Python comparison operators which was taken from w3shool.
Example: Comparison Operators print(3 > 2) # True, because 3 is greater than 2
print(3 >= 2) # True, because 3 is greater than 2 print(3 < 2) # False, because
3 is greater than 2 print(2 < 3) # True, because 2 is less than 3 print(2 <= 3)
# True, because 2 is less than 3 print(3 == 2) # False, because 3 is not equal
to 2 print(3 != 2) # True, because 3 is not equal to 2 print(len('mango') ==
len('avocado')) # False print(len('mango') != len('avocado')) # True
print(len('mango') < len('avocado')) # True print(len('milk') != len('meat')) #
False print(len('milk') == len('meat')) # True print(len('tomato') ==
len('potato')) # True print(len('python') > len('dragon')) # False # Comparing
something gives either a True or False print('True == True: ', True == True)
print('True == False: ', True == False) print('False == False:', False == False)
In addition to the above comparison operator Python uses: • is: Returns true if
both variables are the same object(x is y) • is not: Returns true if both
variables are not the same object(x is not y) • in: Returns True if the queried
list contains a certain item(x in y) • not in: Returns True if the queried list
doesn't have a certain item(x in y) print('1 is 1', 1 is 1) # True - because the
data values are the same print('1 is not 2', 1 is not 2) # True - because 1 is
not 2 print('A in Asabeneh', 'A' in 'Asabeneh') # True - A found in the string
print('B in Asabeneh', 'B' in 'Asabeneh') # False - there is no uppercase B
print('coding' in 'coding for all') # True - because coding for all has the word
coding print('a in an:', 'a' in 'an') # True print('4 is 2 ** 2:', 4 is 2 ** 2)
# True Logical Operators Unlike other programming languages python uses keywords
and, or and not for logical operators. Logical operators are used to combine
conditional statements:
print(3 > 2 and 4 > 3) # True - because both statements are true print(3 > 2 and
4 < 3) # False - because the second statement is false print(3 < 2 and 4 < 3) #
False - because both statements are false print('True and True: ', True and
True) print(3 > 2 or 4 > 3) # True - because both statements are true print(3 >
2 or 4 < 3) # True - because one of the statements is true print(3 < 2 or 4 < 3)
# False - because both statements are false print('True or False:', True or
False) print(not 3 > 2) # False - because 3 > 2 is true, then not True gives
False print(not True) # False - Negation, the not operator turns true to false
print(not False) # True print(not not True) # True print(not not False) # False
Strings Text is a string data type. Any data type written as text is a string.
Any data under single, double or triple quote are strings. There are different
string methods and built-in functions to deal with string data types. To check
the length of a string use the len() method. • Creating a String letter = 'P' #
A string could be a single character or a bunch of texts print(letter) # P
print(len(letter)) # 1 greeting = 'Hello, World!' # String could be made using a
single or double quote,"Hello, World!" print(greeting) # Hello, World!
print(len(greeting)) # 13 sentence = "I hope you are enjoying 30 days of Python
Challenge" print(sentence) Multiline string is created by using triple single
(''') or triple double quotes ("""). See the example below. multiline_string =
'''I am a teacher and enjoy teaching. I didn't find anything as rewarding as
empowering people. That is why I created 30 days of python.'''
print(multiline_string) # Another way of doing the same thing multiline_string =
"""I am a teacher and enjoy teaching. I didn't find anything as rewarding as
empowering people. That is why I created 30 days of python."""
print(multiline_string) • String Concatenation We can connect strings together.
Merging or connecting strings is called concatenation. See the example below:
first_name = 'Asabeneh' last_name = 'Yetayeh' space = ' ' full_name = first_name
+ space + last_name print(full_name) # Asabeneh Yetayeh # Checking the length of
a string using len() built-in function print(len(first_name)) # 8
print(len(last_name)) # 7 print(len(first_name) > len(last_name)) # True
print(len(full_name)) # 16 • Escape Sequences in Strings In Python and other
programming languages \ followed by a character is an escape sequence. Let us
see the most common escape characters: \n: new line \t: Tab means(8 spaces) \\:
Back slash \': Single quote (') \": Double quote (") Now, let us see the use of
the above escape sequences with examples. print('I hope everyone is enjoying the
Python Challenge.\nAre you ?') # line break print('Days\tTopics\tExercises') #
adding tab space or 4 spaces print('Day 1\t5\t5') print('Day 2\t6\t20')
print('Day 3\t5\t23') print('Day 4\t1\t35') print('This is a backslash symbol
(\\)') # To write a backslash print('In every programming language it starts
with \"Hello, World!\"') # to write a double quote inside a single quote #
output I hope every one is enjoying the Python Challenge. Are you ? Days Topics
Exercises Day 1 5 5 Day 2 6 20 Day 3 5 23 Day 4 1 35 This is a backslash symbol
(\) In every programming language it starts with "Hello, World!" • String
formatting Old Style String Formatting (% Operator) In Python there are many
ways of formatting strings. In this section, we will cover some of them. The "%"
operator is used to format a set of variables enclosed in a "tuple" (a fixed
size list), together with a format string, which contains normal text together
with "argument specifiers", special symbols like "%s", "%d", "%f", "%.number of
digitsf". %s - String (or any object with a string representation, like numbers)
%d - Integers %f - Floating point numbers "%.number of digitsf" - Floating point
numbers with fixed precision # Strings only first_name = 'Asabeneh' last_name =
'Yetayeh' language = 'Python' formated_string = 'I am %s %s. I teach %s'
%(first_name, last_name, language) print(formated_string) # Strings and numbers
radius = 10 pi = 3.14 area = pi * radius ** 2 formated_string = 'The area of
circle with a radius %d is %.2f.' %(radius, area) # 2 refers the 2 significant
digits after the point python_libraries = ['Django', 'Flask', 'NumPy',
'Matplotlib','Pandas'] formated_string = 'The following are python libraries:%s'
% (python_libraries) print(formated_string) # "The following are python
libraries:['Django', 'Flask', 'NumPy', 'Matplotlib','Pandas']" • New Style
String Formatting (str.format) This formatting is introduced in Python version
3. first_name = 'Asabeneh' last_name = 'Yetayeh' language = 'Python'
formated_string = 'I am {} {}. I teach {}'.format(first_name, last_name,
language) print(formated_string) a = 4 b = 3 print('{} + {} = {}'.format(a, b, a
+ b)) print('{} - {} = {}'.format(a, b, a - b)) print('{} * {} = {}'.format(a,
b, a * b)) print('{} / {} = {:.2f}'.format(a, b, a / b)) # limits it to two
digits after decimal print('{} % {} = {}'.format(a, b, a % b)) print('{} // {} =
{}'.format(a, b, a // b)) print('{} ** {} = {}'.format(a, b, a ** b)) # output 4
+ 3 = 7 4 - 3 = 1 4 * 3 = 12 4 / 3 = 1.33 4 % 3 = 1 4 // 3 = 1 4 ** 3 = 64 #
Strings and numbers radius = 10 pi = 3.14 area = pi * radius ** 2
formated_string = 'The area of a circle with a radius {} is
{:.2f}.'.format(radius, area) # 2 digits after decimal print(formated_string)
String Interpolation / f-Strings (Python 3.6+) Another new string formatting is
string interpolation, f-strings. Strings start with f and we can inject the data
in their corresponding positions. a = 4 b = 3 print(f'{a} + {b} = {a +b}')
print(f'{a} - {b} = {a - b}') print(f'{a} * {b} = {a * b}') print(f'{a} / {b} =
{a / b:.2f}') print(f'{a} % {b} = {a % b}') print(f'{a} // {b} = {a // b}')
print(f'{a} ** {b} = {a ** b}') • Python Strings as Sequences of Characters
Python strings are sequences of characters, and share their basic methods of
access with other Python ordered sequences of objects – lists and tuples. The
simplest way of extracting single characters from strings (and individual
members from any sequence) is to unpack them into corresponding variables.
Accessing Characters in Strings by Index In programming counting starts from
zero. Therefore the first letter of a string is at zero index and the last
letter of a string is the length of a string minus one. language = 'Python'
first_letter = language[0] print(first_letter) # P second_letter = language[1]
print(second_letter) # y last_index = len(language) - 1 last_letter =
language[last_index] print(last_letter) # n If we want to start from right end
we can use negative indexing. -1 is the last index. language = 'Python'
last_letter = language[-1] print(last_letter) # n second_last = language[-2]
print(second_last) # o • Slicing Python Strings In python we can slice strings
into substrings. language = 'Python' first_three = language[0:3] # starts at
zero index and up to 3 but not include 3 print(first_three) #Pyt last_three =
language[3:6] print(last_three) # hon # Another way last_three = language[-3:]
print(last_three) # hon last_three = language[3:] print(last_three) # hon •
Reversing a String We can easily reverse strings in python. greeting = 'Hello,
World!' print(greeting[::-1]) # !dlroW ,olleH Skipping Characters While Slicing
It is possible to skip characters while slicing by passing step argument to
slice method. language = 'Python' pto = language[0:6:2] # print(pto) # Pto •
String Methods There are many string methods which allow us to format strings.
See some of the string methods in the following example: 1. capitalize():
Converts the first character of the string to capital letter 2. count(): returns
occurrences of substring in string, count(substring, start=.., end=..). The
start is a starting indexing for counting and end is the last index to count. 3.
endswith(): Checks if a string ends with a specified ending 4. expandtabs():
Replaces tab character with spaces, default tab size is 8. It takes tab size
argument 5. find(): Returns the index of the first occurrence of a substring, if
not found returns -1 6. rfind(): Returns the index of the last occurrence of a
substring, if not found returns -1 7. format(): formats string into a nicer
output 8. index(): Returns the lowest index of a substring, additional arguments
indicate starting and ending index (default 0 and string length - 1). If the
substring is not found it raises a valueError. 9. rindex(): Returns the highest
index of a substring, additional arguments indicate starting and ending index
(default 0 and string length - 1) 10. isalnum(): Checks alphanumeric character
11. isalpha(): Checks if all string elements are alphabet characters (a-z and
A-Z) 12. isdecimal(): Checks if all characters in a string are decimal (0-9) 13.
isdigit(): Checks if all characters in a string are numbers (0-9 and some other
unicode characters for numbers) 14. isnumeric(): Checks if all characters in a
string are numbers or number related (just like isdigit(), just accepts more
symbols, like ½) 15. isidentifier(): Checks for a valid identifier - it checks
if a string is a valid variable name 16. islower(): Checks if all alphabet
characters in the string are lowercase 17. isupper(): Checks if all alphabet
characters in the string are uppercase 18. join(): Returns a concatenated string
19. strip(): Removes all given characters starting from the beginning and end of
the string 20. replace(): Replaces substring with a given string 21. split():
Splits the string, using given string or space as a separator 22. title():
Returns a title cased string 23. swapcase(): Converts all uppercase characters
to lowercase and all lowercase characters to uppercase characters 24.
swapcase(): Converts all uppercase characters to lowercase and all lowercase
characters to uppercase characters
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