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.