Python Indentation

Python Expression

An expression is any piece of program code, which returns a value (including None) after being executed

Expressions can be a single value or can consist of literals (strings, numbers, lists, sets, tuples), variables, operators, and functions

The operators and functions may be built-in or user defined

4
3 + 3
[32, "a string literal inside a list"]
str(len("this string is an expression in itself while using len() to get its length can be also considered as an expression") -1) + " + 1"

Each of the lines in above example is an expression

Python Statements

Statements are instructions that a Python interpreter can execute and are often composed of expressions (and/or other statements)

In interactive mode, the value of a statement containing only expressions is converted to string and displayed, but not while executing a Python file

Examples

x = 4
print(3 + 3)
someList = [x, 32, "a string literal inside a list"]

Simple Statements

A simple statement is comprised within a single logical line

Several simple statements may occur on a single line separated by semicolons

Simple statements can be an expression statement, assignment statement, break, del, return statement etc

Compound Statements

Compound statements contain (groups of) other statements and affect or control the execution of those other statements in some way

In general, compound statements span multiple lines, although in some cases a compound statement may be contained in one line

if, while, for , try etc are compound statements

A compound statement consists of one or more clauses

Clause, header and suite in Python

A clause consists of a header and a suite (eg if clause, else clause )

Each clause header begins with a uniquely identifying keyword and ends with a colon

A suite is a group of statements controlled by a clause, which can be one or more indented statements on subsequent lines or can be one or more semicolon-separated simple statements on the same line as the header

The clause headers of a particular compound statement are all at the same indentation level

if x % 2 == 0:
    print("even")
else:
    print("odd")

In above example, the line if x % 2 == 0: is the clause header for the if clause which belongs to the if-else compound statement

The statement print("even") is the suite for the if clause

Indentation in Python refers to the space characters (spaces and tabs) that are used at the beginning of a statement

Programming languages like C, Java, Javascript use braces { } to define a block of code

Python uses indentation to recognize blocks of code or statements which belong to a compound statement (or clauses of a compound statement)

Statements with the same indentation belong to the same suite if there are no statements in between with lesser amount of indentation

Consider following Python code which defines a function to display (or print) some words to the screen, when executed

def display_function():
    print("string from a function")
display_function()

Here, the line print("string from a function") is indented with 4 spaces (i.e., the actual statement is typed after keeping 4 spaces at the beginning of the line)

This is done so that when Python executes this program(or code), it recognizes that line to be part of the function display_function()

Such indented lines are called a suite(or code block) which belong to the recent clause of a compound statement

In case, indentation is of 0 spaces, then these statements are considered global statements, which get executed when the file is run or imported

The function definition does not execute the function body; this gets executed only when the function is called

For example, when python parses or executes the code provided above, it stores the display_function() in memory as a function object with the print() statement as its suite, without executing the print statement

It then moves on to parse (execute) the next dedented line (i.e. the 3rd line above), and as a result executes the print() statement

Indentation is required for statements belonging to a clause or compound statement (like if, while)

Consider following Python code containing a function which takes a number and checks if the number is a whole number, negative number etc

def number_check(n):
    if n == int(n):
        if n >= 0:
            print(n, "is whole number")
        else :
            print(n, "is a negative number")
    else:
        print(n, "is not an integer")

number_check(20)
number_check(20.0001)
number_check(-20)

Output

20 is whole number
20.0001 is not an integer
-20 is negative number

In the above example the 2nd line if n == int(n): and 7th line else: are in the same suite since they have the same indentation

Similarly, the 3rd and 5th lines represent the suite for the 2nd line, and furthermore have their own blocks

For any input, the execution flow first executes the if condition if n == int(n):

If the number is an integer then it moves to the corresponding block and executes the condition if n>= 0

If it evaluates to true then its corresponding print statement print(n, "is whole number") is executed and corresponding else statement is skipped

Similarly the outer else is also skipped (for cases where its corresponding if statement evaluates to True)

Following example shows some indentation errors

     def perm(l):                       # error: first line indented
    for i in range(len(l)):             # error: not indented
        s = l[:i] + l[i+1:]
            p = perm(l[:i] + l[i+1:])   # error: unexpected indent
            for x in p:
                    r.append(l[i:i+1] + x)
                return r                # error: inconsistent dedent

Python Packages