Learning Python

Getting started with Python

At first launch your Python terminal as explained in the above part.

The >>> means that Python is ready and we can enter a command. The basic idea is really simple: we enter a command, press Enter, enter another command, press Enter and keep going.

You probably don’t know any Python commands yet. Let’s see what happens if we just write something and press Enter.

>>> hello

Oops! That didn’t work. But we get an error message that tells us what’s wrong.

Let’s move further.

Maybe we can press Enter without typing anything?

>>>
>>>

That worked. How about numbers?

>>> 123
123
>>> -123
-123
>>> 3.14
3.14
>>> -12.3
-12.3
>>>

As we can see the echoes come back. Hence it worked!

Comments

Comments are text that does nothing. They can be created by typing a # and then some text after it, and they are useful when our code would be hard to understand without them.

>>> 1 + 2     # comment here
3

Python does not really have a syntax for multi line comments. To add a multi-line comment you could insert a # for each line:

>>> # comment 1st Line
>>> # comment 2nd Line
>>> 1 + 2
3

Strings

Strings are small pieces of text that we can use in our programs. We can create strings by simply writing some text in quotes.

>>> 'hello world'
'hello world'

Strings can also be written with “double quotes” instead of ‘single quotes’. This is useful when we need to put quotes inside the string.

Strings can be joined together easily with + or repeated with *:

>>> "hello" + "world"
'helloworld'>>> "hello" * 3
'hellohellohello'

Using Python as a calculator

Let’s type some math stuff into Python and see what it does.

>>> 17 + 3
20
>>> 17 - 3
14
>>> 17 * 3
51
>>> 17 / 3
5.666666666666667

Python does follow “Bodmas rule”. Starting from * Parentheses * Exponents * Multiplication * Division * Addition * Subtraction.

>>> a = 1   # create a variable called a that points to 1
>>> b = 2   # create another variable

>>> a       # get the value that the variable points to
1
>>> b
2

>>> #Legal variable names:
>>> myvar = "John"
>>> my_var = "John"
>>> _my_var = "John"
>>> myVar = "John"
>>> MYVAR = "John"
>>> myvar2 = "John">>> #Illegal variable names:
>>> 2myvar = "John"
>>> my-var = "John"
>>> my var = "John"

>>> a = 1
>>> a = a + 1
>>> a
2

>>> a += 2          # a = a + 2
>>> a -= 2          # a = a - 2
>>> a *= 2          # a = a * 2
>>> a /= 2          # a = a / 2

>>> a = 'hello'
>>> a *= 3
>>> a += 'world'
>>> a
'hellohellohelloworld'

>>> a = 1
>>> a == 1
True
>>> a = 2
>>> a == 1
False

>>> not_val = None
>>> not_val
>>>

>>> x = 1
>>> y = 2
>>> x + y # Addition
3
>>> x - y # Subtraction
-1
>>> x * y # Multiplication
2
>>> x / y # Division
0.5
>>> x % y # Modulus
1
>>> x ** y # Exponentiation
1
>>> x // y # Floor division
0

>>> x = 5
>>> x += 3
>>> x
8
>>> x -= 3
>>> x
5
>>> x *= 3
>>> x
15
>>> x /= 3
>>> x
5.0
>>> x %= 3
>>> x
2.0
>>> x = 5
>>> x //= 3
>>> x
1
>>> x **= 3
>>> x
1
>>> x = 5
>>> x &= 3
>>> x
1
>>>
>>> x |= 3
>>> x
3
>>> x ^= 3
>>> x
0
>>> x >>= 3
>>> x
0
>>> x <<= 3
>>> x
0

>>> x = 2
>>> y = 3
>>> x == y # Equal
False
>>> x != y # Not equal
True
>>> x > y # Greater than
False
>>> x < y # Less than
True
>>> x >= y # Greater than or equal to 
False
>>> x <= y # Less than or equal to
True

>>> x = 9
>>>
>>> # and - Returns True if both statements are true
... x < 5 and  x < 10
False
>>>
>>> # or - Returns True if one of the statements is true
... x < 5 or x < 4
False
>>>
>>> # not - Reverse the result, returns False if the result is true
... not(x < 5 and x < 10)
True

>>> x = 9
>>> y = 9
>>>
>>> # is - Returns True if both variables are the same object 
... x is y
False
>>>
>>> # is not  - Returns True if both variables are not the same object
... x is not y
False

Built-in Function “print”

Now we know how to make Python show text.

>>> 'Hello!'
'Hello!'

But that includes quotes''. One way to show text to the user without quotes'' is with the print function. In Python, printing doesn’t have anything to do with physical printers, it just means showing text on the screen.

>>> print('Hello!')
Hello!>>> print("Hello World!")
Hello World!

Let’s see what happens if we type print without the ('Hello') part.

As we can see, print is a function. Functions do something when they are called by typing their name and parentheses. Inside the parentheses, we can pass some arguments too. In print("hello") the function is print and we give it one argument, which is "hello".

Functions are easy to understand, They simply do something when they are called. Functions run immediately when we call them, so the text appears on the screen right away when we run print(something).

Thing that you could do wrong: Sometimes people think that doing thingy = print('hello') means that Python is going to print hello every time we type thingy. But this is not correct! print('hello') runs print right away, and if we type thingy later it’s not going to run print('hello') again.

Handy things about print

We can also print an empty line by calling print without any arguments.

>>> print()>>>

In Python, \n is a newline character. Printing a string that contains a newline character also prints a newline:

>>> print('hello\nworld')
hello
world
>>>

If we want to print a real backslash, we need to escape it by typing two backslashes.

>>> print('hello\\nworld')
hello\nworld
>>>

We can also pass multiple arguments to the print function. We need to separate them with commas and print will add spaces between them.

>>> print("Hello", "World!")
Hello World!
>>>

Unlike with +, the arguments don’t need to be strings.

>>> print(42, "is an integer, and the value of pi is", 3.14)
42 is an integer, and the value of pi is 3.14
>>>

if, else and elif

Using if statements

An “if statement” is written by using the if keyword.

>>> a = 33
>>> b = 200
>>> if b > a:
...     print("b is greater than a") #indented line
...
b is greater than a
>>>

In this example we use two variables, a and b, which are used as part of the if statement to test whether b is greater than a. As a is 33, and b is 200, we know that 200 is greater than 33, and so we print to screen that “b is greater than a”.

An important thing to notice is that the line with a print is indented. You can press the tab key, or if it doesn’t work just press space a few times.

Python relies on indentation (whitespace at the beginning of a line) to define scope in the code. Other programming languages often use curly-brackets for this purpose.

Also Note: The prompt changed from >>> to .... It meant that Python was expecting me to keep typing. When I was done, I just pressed Enter twice. My code was executed and the prompt went back to >>>.

Using elif statements

The elif keyword is pythons way of saying “if the previous conditions were not true, then try this condition”.

>>> a = 33
>>> b = 200
>>> if b > a:
...     print("b is greater than a") #indented line
...
b is greater than a
>>>

In this example a is equal to b, so the first condition is not true, but the elif condition is true, so we print to screen that “a and b are equal”.

Using else statements

The else keyword catches anything which isn’t caught by the preceding conditions.

>>> a = 200
>>> b = 33
>>> if b > a:
...     print("b is greater than a")
... elif a == b:
...     print("a and b are equal")
... else:
...     print("a is greater than b")
...
a is greater than b
>>>

In this example a is greater than b, so the first condition is not true, also the elif condition is not true, so we go to the else condition and print to screen that “a is greater than b”.

You can also have an else without the elif:

>>> a = 200
>>> b = 33
>>> if b > a:
...     print("b is greater than a")
... else:
...     print("a is greater than b")
...
a is greater than b
>>>

Playing with Strings

Concatenate

>>> "I said: " + our_string
'I said: Hello World!'
>>>

Multiplier for string

>>> our_string = "Hello World!"
>>> our_string * 3
'Hello World!Hello World!Hello World!'
>>>

Thing You Should Know:

Python strings are immutable. That’s just a fancy way to say that they cannot be changed in-place, and we need to create a new string to change them. Even some_string += another_string creates a new string. Python will treat that as some_string = some_string + another_string, so it creates a new string but it puts it back to the same variable.

Slicing

Slicing is really simple. It just means getting a part of the string. For example, to get all characters between the second place between the characters and the fifth place between the characters, we can do this

>>> our_string = "Hello World!"
>>> our_string[2:5]
'llo'
>>>

So the syntax is like some_string[start:end].

But what happens if we slice with negative values?

>>> our_string = "Hello World!"
>>> our_string[-5:-2]
'orl'
>>>

If we don’t specify the beginning it defaults to 0, and if we don’t specify the end it defaults to the length of the string. For example, we can get everything except the first or last character like this:

>>> our_string = "Hello World!"
>>> our_string[1:] 
'ello World!'
>>> our_string[:-1] 
'Hello World'
>>>

Indexing

Indexing strings also starts at zero. The first character is our_string[0], the second character is our_string[1], and so on.

>>> our_string = "Hello World!"
>>> our_string[0]
'H'
>>> our_string[1]
'e'
>>> our_string[2]
'l'
>>> our_string[3]
'l'
>>> our_string[4]
'o'
>>>

String Methods

Python’s strings have many useful methods. The official documentation covers them all, but I’m going to just show some of the most commonly used ones briefly.

Here’s an example with some of the most commonly used string methods:

The upper() method returns the string in upper case:

>>> our_string = "Hello World!"
>>> our_string.upper()
'HELLO WORLD!'

The lower() method returns the string in lower case:

>>> our_string = "Hello World!"
>>> our_string.lower()
'hello world!'

The startswith() method returns True if the string starts with the specified value, otherwise False.

>>> our_string = "Hello World!"
>>> our_string.startswith('Hello')
True

The endswith() method returns True if the string ends with the specified value, otherwise False.

>>> our_string = "Hello World!"
>>> our_string.endswith('World!')
True
>>> our_string.endswith('world!')  # Python is case-sensitive
False

The replace() method replaces a specified phrase with another specified phrase.

>>> our_string = "Hello World!"
>>> our_string.replace('World', 'there')
'Hello there!'

The lstrip() method removes any leading characters (space is the default leading character to remove)

>>> our_string = "Hello World!"
>>> '  hello 123  '.lstrip()    # left strip
'hello 123  '

The rstrip() method removes any trailing characters (characters at the end a string), space is the default trailing character to remove.

>>> our_string = "Hello World!"
>>> '  hello 123  '.rstrip()    # right strip
'  hello 123'

The strip() method removes any whitespace from the beginning or the end:

>>> our_string = "Hello World!"
>>> '  hello 123  '.strip()     # strip from both sides
'hello 123'

The count() method returns the number of times a specified value appears in the string.

>>> our_string = "Hello World!"
>>> our_string.count('o')   # it contains two o's
2

The index() method finds the first occurrence of the specified value.

>>> our_string = "Hello World!"
>>> our_string.index('o')   # the first o is our_string[4]
4

The rindex() method finds the last occurrence of the specified value.

>>> our_string = "Hello World!"
>>> our_string.rindex('o')  # the last o is our_string[7]
7

The join() method takes all items in an iterable and joins them into one string.

>>> '-'.join(['hello', 'world', 'test'])
'hello-world-test'

The split() method splits the string into substrings if it finds instances of the separator.

>>> 'hello-world-test'.split('-')
['hello', 'world', 'test']

String Formatting

The format() method formats the specified value(s) and insert them inside the string’s placeholder.

The placeholder is defined using curly brackets: {}. Read more about the placeholders in the Placeholder section below.

The format() method returns the formatted string.

>>> txt1 = "My name is {fname}, I'am {age}".format(fname = "John", age = 36)
>>> txt1
"My name is John, I'am 36"

Another example:

>>> txt2 = "My name is {0}".format("John")
>>> txt2
'My name is John'

Lists and tuples

Sometimes we may end up doing something like this.

name1 = 'john'
name2 = 'doe'
name3 = 'amin'
name4 = 'mark'
name5 = 'spark'

This code works just fine, but there’s a problem. The name check is repetitive, and adding a new name requires adding even more repetitive. This is where LIST comes into play.

Lists

Instead of adding a new variable for each name it might be better to store all names in one variable. This means that our one variable needs to point to multiple values. An easy way to do this is using a list.

By definition, a list is a collection which is ordered and changeable. In Python lists are written with square brackets.

>>> names = ['john', 'doe', 'amin', 'mark', 'spark']

You access the list items by referring to the index number:

>>> names = ['john', 'doe', 'amin', 'mark', 'spark']
>>> names[1]  
'doe'

Negative indexing means beginning from the end, -1 refers to the last item, -2 refers to the second last item etc.

>>> names = ['john', 'doe', 'amin', 'mark', 'spark']
>>> names[-1]  
'spark'

You can specify a range of indexes by specifying where to start and where to end the range.

>>> names = ['john', 'doe', 'amin', 'mark', 'spark']
>>> names[1:3]  
['doe', 'amin']

Note: The search will start at index 1 (included) and end at index 3 (not included) which is why you are seeing doe, amin and not mark.

To change the value of a specific item, refer to the index number:

>>> names = ['john', 'doe', 'amin', 'mark', 'spark']
>>> names[1] = "smith"
>>> names
['john', 'smith', 'amin', 'mark', 'spark']

To determine how many items a list has, use the len() function:

>>> names = ['john', 'doe', 'amin', 'mark', 'spark']
>>> len(names)
5

To add an item to the end of the list, use the append() method:

>>> names = ['john', 'doe', 'amin', 'mark', 'spark']
>>> names.append('medium')

To add an item at the specified index, use the insert() method:

>>> names = ['john', 'doe', 'amin', 'mark', 'spark']
>>> names.insert(1,'medium')

To add an item at the specified index, use the insert() method:

>>> names = ['john', 'doe', 'amin', 'mark', 'spark']
>>> names.insert(1,'medium')

The remove() method removes the specified item:

>>> names = ['john', 'doe', 'amin', 'mark', 'spark']
>>> names.remove('doe')

The del keyword removes the specified index:

>>> names = ['john', 'doe', 'amin', 'mark', 'spark']
>>> del names[0] #remove john
>>> names 
['doe', 'amin', 'mark', 'spark']

The del keyword can also delete the list completely:

>>> names = ['john', 'doe', 'amin', 'mark', 'spark']
>>> del names

The clear() method empties the list:

>>> names = ['john', 'doe', 'amin', 'mark', 'spark']
>>> names.clear()

Copying a List: You cannot copy a list simply by typing list2 = list1, because: list2 will only be a reference to list1, and changes made in list1 will automatically also be made in list2. You can make a copy of a list with the copy() method:

>>> names = ['john', 'doe', 'amin', 'mark', 'spark']
>>> newnamelist = names.copy()
>>> newnamelist
['john', 'doe', 'amin', 'mark', 'spark']

Join Two Lists: Though there are several ways

>>> list1 = ["a", "b" , "c"]
>>> list2 = [1, 2, 3]
>>> list3 = list1 + list2
>>> list3
['a', 'b', 'c', 1, 2, 3]

** There is one thing we are leaving here Loop Through a List. This we will cover while learning Loops.

Tuples

Tuples are a lot like lists, but they’re immutable so they can’t be changed in-place. We create them like lists, but with () instead of [].

Hence once a tuple is created, you cannot add items to it. Tuples are unchangeable.

>>> numbers = (1, 2, 3)
>>> numbers
(1, 2, 3)

You can access tuple items by referring to the index number, inside square brackets:

>>> numbers = (1, 2, 3)
>>> numbers[1]
2

Negative indexing means beginning from the end, -1 refers to the last item, -2 refers to the second last item etc.

>>> numbers = (1, 2, 3)
>>> numbers[-1]
3

Range of Indexes: You can specify a range of indexes by specifying where to start and where to end the range. When specifying a range, the return value will be a new tuple with the specified items.

>>> thistuple = ("apple", "banana", "cherry", "orange", "kiwi", "melon", "mango")
>>> print(thistuple[2:5])
('cherry', 'orange', 'kiwi')

Note: The search will start at index 2 (included) and end at index 5 (not included). Also remember that the first item has index 0.

Change Tuple Values : Once a tuple is created, you cannot change its values. Tuples are unchangeable, or immutable as it also is called. But there is a workaround. You can convert the tuple into a list, change the list, and convert the list back into a tuple.

>>> x = ("apple", "banana", "cherry")
>>> y = list(x)
>>> y[1] = "kiwi"
>>> x = tuple(y)
>>> print(x)
('apple', 'kiwi', 'cherry')

Tuple Length: To determine how many items a tuple has, use the len() method:

>>> x = ("apple", "banana", "cherry")
>>> print(len(x))
3

Tuples are unchangeable, so you cannot remove items from it, but you can delete the tuple completely:

>>> thistuple = ("apple", "banana", "cherry")
>>> del thistuple

Join Two Tuples: To join two or more tuples you can use the + operator:

>>> tuple1 = ("a", "b" , "c")
>>> tuple2 = (1, 2, 3)
>>> tuple3 = tuple1 + tuple2
print(tuple3)

** There is one thing we are leaving here Loop Through a Tuple. This we will cover while learning Loops.

Loops

In programming, a loop means repeating something multiple times. There are different kinds of loops:

1. While loops repeat something while a condition is true.
2. For loops repeat something for each element of something.

The While loop

With the while loop we can execute a set of statements as long as a condition is true. Let’s take an example to understand how while loop works.

The program prints i as long as i is less than 6:

>>> i = 1
>>> while i < 6:
>>>  print(i)
>>>  i += 1

Note: remember to increment i, or else the loop will continue forever.

The while loop requires relevant variables to be ready, in this example we need to define an indexing variable, i, which we set to 1.

The break Statement

With the break statement we can stop the loop even if the while condition is true:

Exit the loop when i is 3:

>>> i = 1
>>> while i < 6:
>>>  print(i)
>>>  if i == 3:
>>>    break
>>>  i += 1

The continue Statement

With the continue statement we can stop the current iteration, and continue with the next:

Continue to the next iteration if i is 3:

>>> i = 1
>>> while i < 6:
>>>  print(i)
>>>  if i == 3:
>>>    continue
>>>  i += 1

The For loop

A for loop is used for iterating over a sequence (that is either a list, a tuple, a dictionary, a set, or a string). Let’s take an example to understand how while loop works.

Print each fruit in a fruit list: (***This is where we use the LIST)

>>> fruits = ["apple", "banana", "cherry"]
>>> for x in fruits:
>>>   print(x)
apple
banana
cherry

Note: Don’t forget the indentation at print(x) statement.

The break Statement

With the break statement we can stop the loop before it has looped through all the items:

>>> fruits = ["apple", "banana", "cherry"]
>>> for x in fruits:
>>>  print(x)
>>>  if x == "banana":
>>>    break

The continue Statement

With the continue statement we can stop the current iteration of the loop, and continue with the next:

>>> fruits = ["apple", "banana", "cherry"]
>>> for x in fruits:
>>>   if x == "banana":
>>>     continue
>>>   print(x)

Nested Loops

A nested loop is a loop inside a loop. The “inner loop” will be executed one time for each iteration of the “outer loop”:

>>> adj = ["red", "big", "tasty"]
>>> fruits = ["apple", "banana", "cherry"]
>>> for x in adj:
>>>   for y in fruits:
>>>     print(x, y)

Loop Through a List

You can loop through the list items by using a for loop:

>>> thislist = ["apple", "banana", "cherry"]
>>> for x in thislist:
>>>   print(x)

Loop Through a Tuple

You can loop through the tuple items by using a for loop.

>>> thistuple = ("apple", "banana", "cherry")
>>> for x in thistuple:
>>>    print(x)