Learn python in 7 days: Day 2 - Variables & Data types
Shashank Shetty
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List of contents:
Variables
Variables in Python are similar in many ways to
variables in mathematics. Variable is a character(s) that acts as a
placeholder and contains some value or information. A characteristic
feature of variables is that, they vary! Their value is not fixed.
In Python, variables are defined by assigning value to them using
the = symbol, called as the assignment operator.
They can be used in place of the value they contain, i.e., they
behave as if they are indifferent from their value.
x = 7 # The variable x is defined with the value of 7
y = 10 # The variable y is defined with the value of 10
print(x) # Gives the same output as print(7)
print(x + y) # Gives the same output as print(7 + 10)
x = 5 # Changes the value of x from 7 to 5
print(y - x) # Gives the same output as print(10 - 5)
Variables need not be single letter. It is important to note that
although variable names may contain numbers, they cannot begin with
one. Underscores are allowed and later versions of Python also allow
you to use special unicode characters such as π;
yet, some special keywords are reserved (class, def, is, in, not,
etc.) and trying to name a variable using them will result in an
error. Variables in Python are case sensitive, meaning
result is different from Result or
REsulT, which can be both useful and annoying.
Though rarely used, variables can also be deleted using
del. Another variable with the same can be created
afterwards.
strength = 87 # Creates a variable strength with the value of 87
print(strength)
del strength # Deletes the variable strength
print(strength) # Gives an error, because strength is not defined any more
Data Types
Not only numbers, variables can be given any value. Try the following code:
name = Ashok Kumar
print(name)
The above code should give an error! Because python doesn't know
what
Ashok Kumar is. However, enclosing the word in quotes
(single or double) will make it a string and Python doesn't
care what the contents of a string are (for most cases, there are
some special characters which you'll get to know). Now try to run
the above code, but with name = "Ashok Kumar"
Values in Python must be ones that belong to a data type.
type(data) is used to get the data type of a value in
Python. There are numerous data types in Python, but we are
concerned about the basic built-in data types. They are:
Numeric types: int, float, complex
Sequence types: str, list, tuple
Mapping type: dict
Set types: set, frozenset
Boolean type: bool
None type: NoneType
Integer
As the name suggests, the int type is the data type of
integers, simple as that. E.g., 7, 100, 0, -46, 11 + 5
Float
Decimal numbers upto a precision of 15 decimal digits are stored as
float. E.g., 3.7, 0.0628, 4 - 8.33, 2.997e8
Complex
The mathematics students among you might know that the root of -1 is
i and numbers containing it are termed as complex numbers.
The only difference is that the letter j is used in its
place. E.g., 5 + 2j, 0 - 4j, 0j
String
String type is just text, and it can contain unicode characters like
π, α and λ. str is defined by
surrounding the characters by either single or double quotes, but
not a combination of the two. E.g., '', "Hello", 'Python is
awesome!', "It's so cool", "θ"
List
Unsurprisingly, a list is a list of values. It is
defined using square brackets and the elements are separated by
commas. Lists can have elements of different types; they can have
duplicates; they are mutable (modifiable). E.g., [0, 1, 2, 3], [7,
'Apple', 9j, ['a', 'b', 'c']]
Tuple
A tuple is basically a list, but it is immutable,
meaning the values of elements cannot be changed. Elements cannot be
added or removed either. They are defined by round brackets and the
elements are separated by commas. Why would you want to use them?
They are much more efficient in storing information. They can have
duplicates; they can contain different types of elements. E.g., (7,
2025, 4, 899), ('Shashank', 1, 0.005)
Dictionary
A dict in Python holds pairs of information in the
format key:value. The values can be accessed by the
keys with the syntax dict[key]. They can also contain
different data types; they are mutable; but cannot contain duplicate
keys. They are declared in curly brackets and the pairs are
separated by commas. E.g., {0: 89, 1: 46, 2: 99, 3: 95}, {"name":
"Shashank", "age": 103, "height": 2.2, 60: "seconds"}
Set
A set is basically a very efficient list. It cannot
contain duplicates, if there are two elements with the same value,
only one will be stored. They are unordered/not indexed; they can
contain elements of different types; the elements are immutable, but
you can add (set.add(element)) or remove
(set.remove(element)) them. They are defined by curly
brackets, with elements separated by commas.. E.g., {"Ruby", 3.1415,
"Millie", (x, y, z)}, {10, 20, 10, 30, 40}
Frozenset
A frozenset is almost indifferent from a regular set
except that it is immutable as well. It is defined with the syntax
frozenset({elements})
Boolean
There are only two bool values: True and
False. Think of it as Yes or No. They will be useful in
conditional statements. In fact, True is just a placeholder for 1
and False for 0; they are synonymous.
NoneType
The NoneType is a special type in Python which is used
to represent the absence of a value. It is similar to null in other
programming languages. It is the data type of None and
is not the same as 0. 0 is an integer whereas None is... literally
nothing! This is used for debugging and safely handling empty data.
You can convert between data types using the target type as a
function. There are limits of course, E.g., str(111) is
valid whereas int("Hi") is not. Try different
combinations for yourself.
More to know
- Python is a dynamically typed language, meaning you do not have to specify the data type of the value when creating a variable; Python will detect it automatically. Not only that, the data type of the value can be changed as well. However, it is good practice to use type hints, which are basically notes that convey the type of the variable. It is not necessary at all and Python doesn't care about it, in fact, you can label them incorrectly without getting an error; but using them makes the code more readable to others. Type hints can be employed to functions as well and throughout this course, the use of type hints will be encouraged.
score = 99 # Python detects the type of score as int
print(score, type(score))
score = "A+" # Python detects the type of score as str
print(score, type(score))
subject: str = "Science" # Type hinting the variable subject as str
marks: dict[str: int] = {"Physics": 100, "Chemistry": 95, "Biology": 98} # Type hinting the variable marks as dict with str keys and int values
a, b = 5, 10 # Same as a = 5; b = 10
print(a, b)
a, b = b, a # Swaps the values of a and b
print(a, b)
numbers: list[int] = [1, 2, 3, 4, 5] # Creates a list with the elements 1, 2, 3, 4 and 5
print(numbers[0])
text: str = "This is fun!"
print(text[2:7]) # Prints the elements from index 2 to 6 (7 is not included)
print("Apple\nBanana\nCherry") # \n is used to create a new line
print("Apple\tBanana\bCherry") # \t creates a tab and \b removes the previous character
print('ISRO\'s Chandrayaan-3\\Aditya-L1') # Using \ before characters that affect a string stops their effect, including another \.
name: str = "Shashank"
age: int = 16
print(f"Hello, my name is {name} and I am not {age + 10} years old.")
input(). Here is an example:
a: str = input("Enter a number: ") # The input is always a string, so it must be converted to the desired type
b: str = input("Enter another number: ")
print("The sum is: ",int(a) + int(b))