Learn Python

Rules for Python variables:
• 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 (age, Age and AGE are three different variables)
• A variable name cannot be any of the Python keywords

Example
Legal variable names:
myvar="John"
my_var="John"
_my_var="John"
myVar="John"
MYVAR="John"
myvar2="John"

Example
Illegal variable names:
2myvar="John"
my-var="John"
my var="John"

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The Python print() function is often used to output variables.

Example
x="Python is awesome"
print(x)

In the print() function, you output multiple variables, separated by a comma:

Example
x="Python"
y="is"
z="awesome"
print(x, y, z)

You can also use the+ operator to output multiple variables:

Example
x="Python"
y="is"
z="awesome"
print(x+y+z)

Notice the space character after"Python" and"is", without them the result would be"Pythonisawesome".

For numbers, the+ character works as a mathematical operator:

Example
x=5
y=10
print(x+y)

Python will give you an error:

Example (error)
x=5
y="John"
print(x+y)

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Data Type	Example
str	`x = "Hello World"`
int	`x = 20`
float	`x = 20.5`
complex	`x = 1j`
list	`x = ["apple", "banana", "cherry"]`
tuple	`x = ("apple", "banana", "cherry")`
range	`x = range(6)`
dict	`x = {"name" : "John", "age" : 36}`
set	`x = {"apple", "banana", "cherry"}`
frozenset	`x = frozenset({"apple", "banana", "cherry"})`
bool	`x = True`
bytes	`x = b"Hello"`
bytearray	`x = bytearray(5)`
memoryview	`x = memoryview(bytes(5))`
NoneType	`x = None`

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Arithmetic Operators are used to perform mathematical calculations.

Operator	Name	Example	Result
+	Addition	`5+3`	8
-	Subtraction	`10-4`	6
*	Multiplication	`3*4`	12
/	Division	`15/3`	5.0
%	Modulus	`10%3`	1
**	Exponentiation	`2**3`	8
//	Floor Division	`17//5`	3

اختبر فهمك:
ما نتيجة 7+3*2؟

Comparison Operators compare two values and return True or False.

Operator	Description	Example	Result
==	Equal	`5==5`	True
!=	Not Equal	`3!=4`	True
>	Greater Than	`7>10`	False
<	Less Than	`2<5`	True
>=	Greater or Equal	`5>=5`	True
<=	Less or Equal	`3<=2`	False

اختبر فهمك:
ما نتيجة 10!=10؟

Logical Operators combine conditional statements.

Operator	Description	Example
and	Returns True if both statements are true	`5>3 and 2<4` → True
or	Returns True if one of the statements is true	`5>10 or 2<4` → True
not	Reverse the result	`not(5>10)` → True

اختبر فهمك:
ما نتيجة True and False؟

Identity Operators compare objects(memory location), not values.

Operator	Description	Example
is	Returns True if both variables are the same object	`x=[1,2]; y=x; x is y` → True
is not	Returns True if both variables are different objects	`x=[1,2]; y=[1,2]; x is not y` → True

ملاحظة: لا تخلط بين ==(المساواة) و is(هوية الكائن)!

اختبر فهمك:
إذا كان a=[1,2] و b=[1,2]، ما نتيجة a is b؟

Assignment Operators assign values to variables.

Operator	Example	Same As	Effect
=	`x=5`	-	Assign 5 to x
+=	`x+=3`	`x=x+3`	Add then assign
-=	`x-=2`	`x=x-2`	Subtract then assign
*=	`x*=4`	`x=x*4`	Multiply then assign
/=	`x/=2`	`x=x/2`	Divide then assign
%=	`x%=3`	`x=x%3`	Modulus then assign

اختبر فهمك:
إذا كان num=10 ثم طبقنا num*=2، كم تصبح قيمة num؟

Indentation refers to the spaces at the beginning of a code line. Where in other programming languages the indentation in code is for readability only, the indentation in Python is very important. Python uses indentation to indicate a block of code.

Example
if 5>2:
print("Five is greater than two!")

Python will give you an error if you skip the indentation:

Example
Syntax Error:
if 5>2:
print("Five is greater than two!")

The number of spaces is up to you as a programmer, the most common use is four, but it has to be at least one.

You have to use the same number of spaces in the same block of code, otherwise Python will give you an error:

Example
Syntax Error:
if 5>2:

    print("Five
                is greater than two!")

print("Five is greater than two!")

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Python supports the usual logical conditions from mathematics:

• Equals: a==b
• Not Equals: a!=b
• Less than: a• Less than or equal to: a<=b
• Greater than: a>b
• Greater than or equal to: a>=b

These conditions can be used in several ways, most commonly in"if statements" and loops.
An"if statement" is written by using the if keyword.

Elif
The elif keyword is Python's way of saying"if the previous conditions were not true, then try this condition".
Example
a=33
b=33
if b>a:

    print("b
                is greater than a")

elif a==b:

    print("a and b
                are equal")

Else
The else keyword catches anything which isn't caught by the preceding conditions.
Example
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")

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:
Example
a=200
b=33

if
                b>a:

print("b is greater than a")
else:

    print("b
                is not greater than a")

Short Hand If
If you have only one statement to execute, you can put it on the same line as the if statement.
Example
One line if statement:
if a>b: print("a is greater than b")

Short Hand If... Else
If you have only one statement to execute, one for if, and one for else, you can put it all on the same line:
Example
One line if else statement:
a=2
b=330

print("A")
                if a>b else print("B")

And
The and keyword is a logical operator, and is used to combine conditional statements:
Example
Test if a is greater than b, AND if c is greater than a:
a=200
b=33
c=500

if
                a>b and c>a:

    print("Both conditions are
                True")

Or
The or keyword is a logical operator, and is used to combine conditional statements:
Example
Test if a is greater than b, OR if a is greater than c:
a=200
b=33
c=500

if a>b or
                a>c:

    print("At least one of the conditions is
                True")

Not
The not keyword is a logical operator, and is used to reverse the result of the conditional statement:
Example
Test if a is NOT greater than b:
a=33
b=200
if not a>b:

    print("a
                is NOT greater than b")

Nested If
You can have if statements inside if statements, this is called nested if statements.
Example
x=41

if
                x>10:

print("Above ten,")

    if
                x>20:

        print("and also above
                20!")

else:

        print("but
                not above 20.")

The pass Statement
if statements cannot be empty, but if you for some reason have an if statement with no content, put in the pass statement to avoid getting an error.
Example
a=33
b=200

if
                b>a:

pass

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A for loop is used for iterating over a sequence(that is either a list, a tuple, a dictionary, a set, or a string).

With the for loop we can execute a set of statements, once for each item in a list, tuple, set etc.

Example
Print each fruit in a fruit list:
fruits=["apple","banana","cherry"]

for
                             
x in fruits:

print(x)

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

1 Example
Exit the loop when x is"banana":
fruits=["apple","banana","cherry"]

for
                        x in fruits:

print(x)
if x=="banana":
break

2 Example
Exit the loop when x is "banana", but this time the break comes before the print:
fruits=["apple","banana","cherry"]

for x in
                        fruits:

if x=="banana":
break
print(x)

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

Example
Do not print banana:
fruits=["apple","banana","cherry"]

for
                        x in fruits:

if x=="banana":
continue
print(x)

The else keyword in a for loop specifies a block of code to be executed when the loop is finished:

Example
Print all numbers from 0 to 5, and print a message when the loop has ended:
for x in range(6):
print(x)
else:

print("Finally
                        finished!")

With the while loop we can execute a set of statements as long as a condition is true.

Example
Print i as long as i is less than 6:
i=1
while i<6:
print(i)
i+=1

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

Example
Exit the loop when i is 3:
i=1
while i<6:
    print(i)
    if i==3:
        break
    i+=1

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

Example
Continue to the next iteration if i is 3:
i=0
while i<6:
    i+=1
    if i==3:
        continue
    print(i)

With the else statement we can run a block of code once when the condition no longer is true:

Example
Print a message once the condition is false:
i=1
while i<6:
    print(i)
    i+=1
else:
    print("i is no longer less than 6")

Lists are used to store multiple items in a single variable.

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There are several methods to add items to a list in Python. The most common methods are append() and insert().

append() - Adds an element to the end of the list.

Syntax:
list.append(element)

Example:
fruits = ["apple", "banana", "cherry"]
fruits.append("orange")
print(fruits)
Output: ["apple", "banana", "cherry", "orange"]

insert() - Adds an element at a specified position.

Syntax:
list.insert(index, element)

Example:
fruits = ["apple", "banana", "cherry"]
fruits.insert(1, "orange")
print(fruits)
Output: ["apple", "orange", "banana", "cherry"]

extend() - Adds all elements from an iterable to the end of the list.

Syntax:
list.extend(iterable)

Example:
fruits = ["apple", "banana", "cherry"]
tropical = ["mango", "pineapple", "papaya"]
fruits.extend(tropical)
print(fruits)
Output: ["apple", "banana", "cherry", "mango", "pineapple", "papaya"]

There are several methods to remove items from a list in Python.

remove() - Removes the first occurrence of a specified value.

Syntax:
list.remove(value)

Example:
fruits = ["apple", "banana", "cherry"]
fruits.remove("banana")
print(fruits)
Output: ["apple", "cherry"]

pop() - Removes and returns the element at the specified index.

Syntax:
list.pop(index)

Example:
fruits = ["apple", "banana", "cherry"]
fruits.pop(1)
print(fruits)
Output: ["apple", "cherry"]

clear() - Removes all elements from the list.

Syntax:
list.clear()

Example:
fruits = ["apple", "banana", "cherry"]
fruits.clear()
print(fruits)
Output: []

Python lists have a built-in sort() method that modifies the list in-place.

sort() - Sorts the list in ascending order by default.

Syntax:
list.sort()

Example:
fruits = ["banana", "apple", "cherry"]
fruits.sort()
print(fruits)
Output: ["apple", "banana", "cherry"]

Use the reverse=True parameter to sort in descending order.

Syntax:
list.sort(reverse=True)

Example:
numbers = [100, 50, 65, 82, 23]
numbers.sort(reverse=True)
print(numbers)
Output: [100, 82, 65, 50, 23]

Complete List of Python List Methods:

Method	Description
`append()`	Adds an element to the end
`clear()`	Removes all elements
`copy()`	Returns a copy of the list
`count()`	Returns count of an element
`extend()`	Adds elements from an iterable
`index()`	Returns index of first occurrence
`insert()`	Adds element at specified position
`pop()`	Removes and returns element at index
`remove()`	Removes first occurrence of value
`reverse()`	Reverses the list order
`sort()`	Sorts the list