Ch 5Getting Started with Python — Class 11 Computer Science, concept-first.

A computer can only act on instructions it is given, so before a computer solves a problem for us, we must spell out the solution as a precise sequence of instructions.

Python's popularity as a first programming language comes from a specific set of features, each of which shapes how we write and run programs in it.

To write and execute a Python program, one thing is essential: a Python interpreter. There are two ways to get one —

The Python interpreter can be used in two ways:

Some words in Python are not available for us to use as names — the language has already claimed them.

In programming languages, identifiers are the names used to identify a variable, a function, or any other entity in a program. Choosing them is not free-form — Python lays down precise rules.

A variable in a program is uniquely identified by a name — an identifier. In Python, a variable refers to an object: an item or element stored in the memory.

Comments are remarks or notes added inside source code. The interpreter does not execute comments — they exist purely for the humans reading the program.

Python treats every value or data item as an object — whether it is numeric, a string, or of any other type (the types themselves are discussed in the next section).

Every value in Python belongs to a specific data type. A data type does two jobs at once: it identifies what kind of data values a variable can hold, and it determines which operations can be performe…

The Number data type stores numerical values only. It is further classified into three different types — int, float and complex.

A Python sequence is an ordered collection of items, where each item is indexed by an integer — the position of every element is fixed and numbered.

A set is an unordered collection of items, separated by commas and enclosed in curly brackets . At first glance a set looks similar to a list — both hold multiple comma-separated items — but two pr…

None is a special data type with a single value. It is used to signify the absence of value in a situation — a variable that exists but deliberately holds "nothing yet" can be given the value None.

Mapping is an unordered data type in Python. Where a sequence looks items up by their integer position, a mapping looks values up by a key.

Programs often need to change or update the values of certain variables as they run. For some data types Python allows this freely — but for others, once a variable of that type has been created and a…

Python offers four collection types — list, tuple, set, dictionary — and each has a situation in which it is the preferred choice.

An operator is a symbol that tells Python to carry out a specific mathematical or logical operation on one or more values. The values an operator acts upon are called operands.

Arithmetic operators let a Python program do calculation. Python supports the four basic arithmetic operations — addition, subtraction, multiplication and division — and adds three more: modulus (rema…

A relational operator compares the values of the operands on either side of it and works out the relationship between them.

An assignment operator assigns a value to — or changes the value of — the variable on its left-hand side.

Python supports three logical operators: and, or and not. They must be written in lower case only — AND or Or will not work, because Python is case-sensitive and its keywords are fixed.

Identity operators answer a different question from the equality operator ==. Instead of asking "do these two values look the same?", they ask "is the value of a variable of a certain type?" and, more…

Membership operators are used to check whether a value is a member of a given sequence or not. Instead of comparing two values, they search: "does this item occur anywhere inside that collection?" The…

An expression is a combination of constants, variables and operators. The defining property of an expression is that it always evaluates to a value — Python works it out and produces a single result.

When an expression contains different kinds of operators, Python needs a rule to decide which operator to apply first.

In Python, a statement is a unit of code that the Python interpreter can execute. Where an expression evaluates to a value, a statement is an instruction — something the interpreter carries out.

A program often needs to interact with its user — to collect some input data from the end user, process it, and present the desired output.

Sometimes the data type a value arrives in is not the type our calculation needs. Type conversion is the changing of a variable's data type from one type to another — and a small program shows exactly…

Explicit conversion, also called type casting, happens when data type conversion takes place because the programmer forced it in the program. The general form of an explicit data type conversion is:

Implicit conversion, also known as coercion, happens when data type conversion is done automatically by Python, without any instruction from the programmer.

A programmer can make mistakes while writing a program — and because of those mistakes the program may fail to execute at all, or may run but produce wrong output.

Like every programming language, Python has its own rules that determine its syntax — the grammar its statements must follow.

A logical error is a bug in the program that causes it to behave incorrectly — the program runs, but does the wrong thing.

A runtime error causes abnormal termination of the program while it is executing. It arises when a statement is syntactically correct, but the interpreter still cannot execute it.

- Python is an open-source, high-level, interpreter-based language usable for a multitude of scientific and non-scientific computing purposes.