Python Basics: Tokens, Variables, Types

Why Python for Informatics Practices?

Informatics Practices is all about working with data — collecting it, storing it, analysing it and presenting it. Python is the most popular language for data work because it is simple to read, free, and has powerful libraries such as NumPy (numbers), Pandas (tables) and Matplotlib (charts), which you will use in Classes XI and XII.

Execution modes

>>> 45.5 * 3
136.5
>>> 'IP' * 4
'IPIPIPIP'
>>> 100 / 8
12.5

First program

# Shop bill – our first IP program
item = 'Notebook'
price = 45.5
qty = 3
bill = price * qty
print('Item:', item)
print('Bill: Rs.', bill)

Tokens: the smallest units

Just as a sentence is made of words and punctuation, a Python statement is made of tokens.

Keywords and identifiers

Keywords are reserved words (35 in modern Python) — you cannot use them as names. Identifiers are names for variables, functions and so on. Rules for identifiers: letters, digits and underscore only; cannot start with a digit; cannot be a keyword; case-sensitive.

Literals

A literal is a raw value written directly in a program.

print(0o17, 0x1F, 0b101)          # octal, hex, binary integer literals
print(2.5e3, 3 + 2j)              # float in exponent form, complex
print('It\'s', "say \"hi\"")       # escape quotes with backslash
print('Line1\nLine2\tTabbed')      # \n new line, \t tab
print(True, False, None)          # boolean and special literals

Escape sequences start with a backslash: \n new line, \t tab, \\ backslash, \' and \" quotes.

Variables and assignment

A variable is a name that refers to a value stored in memory. It is created the first time you assign to it. Python is dynamically typed:

score = 50
print(score, type(score))
score = 'fifty'          # same name, now a string (dynamic typing)
print(score, type(score))

a = b = c = 100          # same value to many variables
x, y, z = 'Red', 'Green', 'Blue'
print(a, b, c)
print(x, y, z)
print(id(a) == id(b))    # both names refer to one object

Comments

Comments explain code to humans and are ignored by Python. Start a comment with #. Write comments for formulas and important steps, e.g. # GST at 18% is added to the price.

Data types

item, price, qty, in_stock = 'Notebook', 45.5, 3, True
print(type(item), type(price), type(qty), type(in_stock))
runs = [45, 102, 8]                  # list
player = {'name': 'Virat', 'runs': 102}   # dictionary
location = (28.61, 77.21)            # tuple
print(runs, player['runs'], location[0])
print(type(runs), type(player), type(location))

Mutable types can be changed after creation (list, dictionary, set); immutable types cannot (int, float, bool, str, tuple).

Summary & practice

• Python is the main language for data work (NumPy, Pandas, Matplotlib).
• Tokens: keywords, identifiers, literals, operators, punctuators.
• Variables are created by assignment; Python is dynamically typed.
• Types: int, float, complex, bool, str, list, tuple, dict, set, None.