Brief Overview of Python

Written and reviewed by Rohit Singh, Computer Science & Commerce faculty at UniDrill (MCA, Osmania University · 6 years teaching). Based on NCERT Informatics Practices Class XI, Chapter 3.

Why does one missing bracket stop everything?

A computer follows instructions exactly, with no guessing. Leave out one closing bracket and Python refuses to run the program at all. Leave out the right idea and it runs happily and gives the wrong answer. Learning Python is learning to give exact instructions, and exam questions test the same skill: read a few lines, say what happens, and say what would go wrong.

An ordered set of instructions executed by a computer is a program, and the language used to write it is a programming language. Python is a popular, easy to learn language created by Guido van Rossum in 1991. It is used in software development, web development, scientific computing, big data and Artificial Intelligence. Python is an open source, high level, interpreter based language. The programs in this book use Python 3.

Working with Python: two modes

To write and run a Python program you need a Python interpreter (also called the Python shell). The prompt >>> shows that the interpreter is ready to receive instructions.

In IDLE you choose File, New File, write the program and save it, then use Run, Run Module to execute it. The output appears in the shell.

Keywords and identifiers

Keywords are reserved words that have a fixed meaning for the interpreter. Python is case sensitive, so keywords must be written exactly as defined: True, False and None begin with a capital letter, and words such as if, for, while, and, or, not, in, is, def, class, import and return are all lower case.

Identifiers are the names you give to variables, functions and other entities. The rules:

Worked example: which identifiers are valid? Check Serial_no., 1st_Room, Hundred followed by a dollar sign, Total Marks, Total_Marks, total-Marks, _Percentage, True.

The common wrong step is to accept total-Marks because it "looks readable". Python reads the hyphen as a minus sign, so it can never be part of a name.

Why meaningful names matter. To find an average you could write avg = (marksMaths + marksEnglish + marksIP)/3. Names like marksMaths tell the reader what the data is. Names like a, b and c do not.

Variables and comments

A variable is an identifier whose value can change. You create one with an assignment statement, and the interpreter replaces the variable name with its current value wherever it appears.

A variable must be given a value before it is used, otherwise Python raises an error. A comment begins with # and runs to the end of the line. The interpreter ignores it. It exists only to explain the code to humans.

Worked example: sum of two numbers.

The first two lines store values, the third computes 10 + 20, and print() shows the result. The comment line does nothing.

Data types

Every value has a data type, which says what kind of data a variable can hold and what operations can be done on it. The built in function type() reports it.

For Boolean values, True counts as a non zero value and False is the value zero.

Sequences are ordered collections in which each item is indexed by an integer.

The quotes around a string are not part of it. They only tell the interpreter where the string starts and stops. A dictionary stores key value pairs in curly brackets { }, with a colon between key and value, and you read a value by writing its key in square brackets. Keys are usually strings, and values can be of any type.

Beyond the textbook. For the exam: a dictionary is an unordered mapping data type, and there is only one standard mapping type. In reality: from Python 3.7 onwards a dictionary remembers the order in which items were inserted, though you still look values up by key and never by position. Why it matters: answer "unordered" if the question uses the book's wording. Do not use position to read a value from a dictionary.

Pause and predict: what data type does Python give to type("452"), and can you add 1 to it?

Check your answer: it is a string (str), because the digits sit inside quotes. Adding the number 1 to it raises an error, since numerical operations cannot be done on strings, even numeric looking ones.

Operators

An operator performs a mathematical or logical operation on values. The values it works on are operands. In 10 + num, 10 and num are operands and + is the operator.

Arithmetic operators

The + operator joins two strings (concatenation), so "Hello" + "India" gives 'HelloIndia'. The * operator repeats a string when the other operand is an integer, so 'India' * 2 gives 'IndiaIndia'. Both also work for lists and tuples.

Why // and % come as a pair. Divide 13 by 5. The whole number of fives is 2 (13 // 5) and what is left over is 3 (13 % 5). Check: $5 \times 2 + 3 = 13$. Whenever a question mixes // and %, rebuild the number this way.

Beyond the textbook. For the exam: // gives the quotient with the decimal part removed. In reality: it rounds down (floors), which differs from "removing the decimal part" when a number is negative. -5 // 2 is -3, not -2, and -13 % 5 is 2. Why it matters: CUET questions at this level use positive numbers, so the simple rule works, but knowing floor division prevents surprises in later chapters.

Relational operators compare values and give True or False: == (equal), != (not equal), >, <, >=, <=. Strings are compared lexicographically, character by character using the ASCII values of the characters. So "Good" > "Afternoon" is True, because G comes after A.

Assignment operators store values. = assigns, and the shortcuts +=, -=, *=, /=, %=, //= and **= update a variable. x += y means x = x + y.

Logical operators and, or, not must be in lower case.

Membership operators in and not in check whether a value is present in a sequence. With numSeq = [1, 2, 3], 2 in numSeq is True but '1' in numSeq is False, because the string '1' is not the integer 1.

Expressions and precedence

An expression is a combination of constants, variables and operators, and it always evaluates to a value. A single value or a lone variable counts as an expression, but a standalone operator does not.

When an expression has several operators, precedence decides which one is applied first. Higher precedence goes first. Operators of equal precedence work from left to right. Parentheses override everything, and the part inside ( ) is evaluated first.

Worked example: evaluate step by step. Evaluate 20 + 30 * 40, then (20 + 30) * 40, then 15.0 / 4.0 + (8 + 3.0).

The common wrong step is to work strictly left to right and get $(20 + 30) \times 40 = 2000$ for the first expression.

Worked example: operators and booleans. Let num1 = 4, num2 = 3, num3 = 2. Find the output.

The arithmetic runs first, the relational comparisons come next and and runs last. That is exactly the precedence table in action.

Input and output

The input() function reads what the user types and always returns it as a string, even if the user types digits. The syntax is variable = input([Prompt]), where the prompt is optional text shown on screen. To use the value as a number, convert it with int() (or float()). If the text is not numeric, an error occurs.

The print() function evaluates the expression and displays the result. With several arguments separated by commas, it prints them with a space in between.

Why 25.0 and not 25? One operand is a float (2.5), so the result is a float. Mixed arithmetic of int and float always gives a float.

Pause and predict: a user types 7 when asked to enter a number, and the program runs n = input("Enter: ") followed by print(n + n). What is printed?

Check your answer: 77. The input is the string '7', and + joins two strings. If you wanted 14, you must write int(input("Enter: ")).

Debugging: three kinds of error

Programs fail in different ways. Debugging is identifying and removing errors.

Why logical errors are the hardest. There is no error message. The program looks perfect and the answer is quietly wrong. The fix is (10 + 12) / 2, because division has higher precedence than addition.

Worked example: classify the error. (a) 25 / 0 (b) print("Hi" (c) averaging with a + b / 2.

Functions

A function is a set of statements grouped under a name to perform a task. It is defined once and reused by calling its name. For example, a function CalcCompInt could hold the compound interest steps so you do not rewrite them at every place in the program.

Python has many built in functions, such as print() and input(). A module is a Python file that groups many functions, and you bring one into your program with import. To use a function you need to know three things.

In this program the first line uses two built in functions, int() and input(). The input function gets the text typed by the user, and int() takes that text as its argument and returns an integer. The print() function has four arguments.

Decision making: if, if else, if elif else

Normally statements run one after another. Conditional statements let the program choose a path. Python marks a block of statements by indentation, which is leading whitespace at the start of a line. All statements at the same indentation level belong to one block, and wrong indentation is a syntax error. A colon : ends each condition line.

If the first condition is false, the next is checked, and so on. As soon as one is true, its block runs and the whole if statement ends. The number of elif parts depends on how many conditions you need. If no condition is true, the else block runs.

Worked example: trace an if else. The program subtracts the smaller number from the larger. The user enters 5 and then 6.

The common wrong step is to forget that print is not indented, so it runs after the if else whichever branch was taken.

Why elif and not many separate if statements? With separate if lines, Python checks every condition even after one was true. With elif, only the first true branch runs. That is both faster and avoids two blocks running for one input.

Loops: for and range()

Repeating work is called looping or iteration. The for statement repeats a block for each item in a range of values or a sequence (a string, list or tuple). When the items are used up, the loop ends and the statement after it runs. Use for when you know in advance how many times to repeat.

Syntax: for <control-variable> in <sequence or range>: followed by an indented body.

The range() function has the form range(start, stop, step). It gives integers from start up to, but not including, stop, in jumps of step. If start is missing it is 0, and if step is missing it is 1. All values must be integers, and step can be negative but not zero.

Beyond the textbook. For the exam: range() creates a sequence of integers, and you see it with list(range(...)). In reality: in Python 3 range() does not build the whole list. It produces numbers one at a time when needed, which saves memory, and wrapping it in list() is what turns it into a visible list. Why it matters: typing print(range(5)) alone shows range(0, 5), not the numbers.

Worked example: trace a for with range. Predict the output.

The common wrong step is to include 5 because range(5) "means up to 5". The loop stops before 5, so the last value of num is 4.

Worked example: loop over a string and a range with step.

The first loop prints I, N, D, I, A on separate lines, one character per iteration. The second loop prints 20, 22, 24, 26 and 28 on separate lines, because it jumps by 2 and stops before 30.

Nested loops

A loop inside another loop is a nested loop. For each single pass of the outer loop, the inner loop runs completely.

How to read it. The outer loop runs 3 times and the inner loop runs 2 times per outer pass, so the inner print(var2 + 1) runs $3 \times 2 = 6$ times. The line "Out of inner loop" is indented under the outer loop, so it runs 3 times. The last line has no indent, so it runs once.

Pause and predict: if the inner loop in the program above were range(4), how many times would the number lines be printed in total?

Check your answer: $3 \times 4 = 12$ times. The number of inner runs is the outer count multiplied by the inner count.

Exam traps

60-second revision

Where this chapter connects

• Previous: Emerging Trends mentions Pandas, the Python library for data analysis that Class XII builds on.
• Next: Working with Lists and Dictionaries takes the list and dictionary introduced here and shows how to change and traverse them.
• Class XII link: Data Handling using Pandas I applies these same Python basics to tables of data.