3.1 built-in function

     type(42)=> <class 'int'>

     int('32')=>32

     int(3.9) => 3

     int(-2.3)=>-2

     float(32)=> 32.0

     float('3.14159')=>3.14159

     str(32) => '32'

     str(3.14159)=>'3.14159'

3.2 define a function

Defining a function creates a function object, which has type function.

you have to create a function before you can run it. In other words, the function definition has to run before the function gets called. 

The rules for function names are the same as for variable names: letters, numbers and underscore are legal, but the first character can’t be a number.  

Single quotes and double quotes do the same thing; most people use single quotes except in cases like this where a single quote (which is also an apostrophe) appears in the string. 

Function definitions get executed just like other statements, but the effect is to create function objects. The statements inside the function do not run until the function is called, and the function definition generates no output. 

3.3 flow of execution

Execution always begins at the first statement of the program. Statements are run one at a time, in order from top to bottom.

Function definitions do not alter the flow of execution of the program, but remember that statements inside the function don’t run until the function is called. 

A function call is like a detour in the flow of execution. Instead of going to the next state- ment, the flow jumps to the body of the function, runs the statements there, and then comes back to pick up where it left off. 

3.8 Variables and parameters are local

When you create a variable inside a function, it is local, which means that it only exists inside the function.