Program and program flow

A program is a sequence of statements that are executed in a top-down order. This linear execution order has some important exceptions:

• There might be a conditional execution of alternative groups of statements (blocks), which we refer to as branching.
• There are blocks that are executed repetitively, which is called looping (refer to the following Figure 1.2, Program flow).
• There are function calls that are references to another piece of code, which is executed before the main program flow is resumed. A function call breaks the linear execution and pauses the execution of a program unit while it passes the control to another unit-a function. When this gets completed, its control is returned to the calling unit.

Figure 1.2: Program flow

Python uses a special syntax to mark blocks of statements: a keyword, a colon, and an indented sequence of statements, which belong to the block (refer to the following Figure 1.3,  Block command).

Figure 1.3: Block command

Comments

If a line in a program contains the symbol #, everything following on the same line is considered as a comment:

# This is a comment of the following statement
a = 3  # ... which might get a further comment here  

Line joining

A backslash \ at the end of the line marks the next line as a continuation line, that is, explicit line joining. If the line ends before all the parentheses are closed, the following line will automatically be recognized as a continuation line, that is, implicit line joining.