Dissecting Your Python Program

With your new program open in the file editor, let’s take a quick tour of the Python instructions it uses by looking at what each line of code does.

Dissecting Your Python Program

Comments

The following line is called a comment.

# This program says hello and asks for my name.

Python ignores comments, and you can use them to write notes or remind yourself what the code is trying to do. Any text for the rest of the line following a hash mark (#) is part of a comment.

Sometimes, programmers will put a # in front of a line of code to temporarily remove it while testing a program. This is called commenting out code, and it can be useful when you’re trying to figure out why a program doesn’t work. You can remove the # later when you are ready to put the line back in.

Python also ignores the blank line after the comment. You can add as many blank lines to your program as you want. This can make your code easier to read, like paragraphs in a book.

The print() Function
The print() function displays the string value inside the parentheses on the screen.

print('Hello world!')print('What is your name?') # ask for their name

The line print('Hello world!') means “Print out the text in the string 'Hello world!'.” When Python executes this line, you say that Python is calling the print() function and the string value is being passed to the function.

A value that is passed to a function call is an argument. Notice that the quotes are not printed to the screen. They just mark where the string begins and ends; they are not part of the string value.

Note:You can also use this function to put a blank line on the screen; just call print() with
nothing in between the parentheses.

When writing a function name, the opening and closing parentheses at the end identify it as the name of a function. This is why in this book you’ll see print() rather than print. Chapter 2 describes functions in more detail.

The input() Function

The input() function waits for the user to type some text on the keyboard and press enter.

myName = input()

This function call evaluates to a string equal to the user’s text, and the previous line of code assigns the myName variable to this string value.

You can think of the input() function call as an expression that evaluates to whatever string the user typed in. If the user entered 'Al', then the expression would evaluate to myName = 'Al'.

Printing the User’s Name

The following call to print() actually contains the expression 'It is good to meet you, ' + myName between the parentheses.

print('It is good to meet you, ' + myName)

Remember that expressions can always evaluate to a single value. If 'Al' is the value stored in myName on the previous line, then this expression evaluates to 'It is good to meet you, Al'. This single string value is then passed to print(), which prints it on the screen.

The len() Function

You can pass the len() function a string value (or a variable containing a string), and the function evaluates to the integer value of the number of characters in that string.

print('The length of your name is:')print(len(myName))

Enter the following into the interactive shell to try this:

>>> len('hello')5>>> len('My very energetic monster just scarfed nachos.')46>>> len('')0

Just like those examples, len(myName) evaluates to an integer. It is then passed to print() to be displayed on the screen. Notice that print() allows you to pass it either integer values or string values. But notice the error that shows up when you type the following into the interactive shell:

>>> print('I am ' + 29 + ' years old.')Traceback (most recent call last):File "<pyshell#6>", line 1, in <module>print('I am ' + 29 + ' years old.')TypeError: Can't convert 'int' object to str implicitly

The print() function isn’t causing that error, but rather it’s the expression you tried to pass to print(). You get the same error message if you type the expression into the interactive shell on its own.

>>> 'I am ' + 29 + ' years old.'Traceback (most recent call last):File "<pyshell#7>", line 1, in <module>'I am ' + 29 + ' years old.'TypeError: Can't convert 'int' object to str implicitly

Python gives an error because you can use the + operator only to add two integers together or concatenate two strings. You can’t add an integer to a string because this is ungrammatical in Python. You can fix this by using a string version of the integer instead, as explained in the next section.

The str(), int(), and float() Functions

If you want to concatenate an integer such as 29 with a string to pass to print(), you’ll need to get the value '29', which is the string form of 29. The str() function can be passed an integer value and will evaluate to a string value version of it, as follows:

>>> str(29)'29'>>> print('I am ' + str(29) + ' years old.')I am 29 years old.

Because str(29) evaluates to '29', the expression 'I am ' + str(29) + ' years old.' evaluates to 'I am ' + '29' + ' years old.', which in turn evaluates to 'I am 29 years old.'. This is the value that is passed to the
print() function.

The str(), int(), and float() functions will evaluate to the string, integer, and floating-point forms of the value you pass, respectively. Try converting some values in the interactive shell with these functions, and watch what happens.

>>> str(0)'0'>>> str(-3.14)'-3.14'>>> int('42')42>>> int('-99')-99>>> int(1.25)1>>> int(1.99)1>>> float('3.14')3.14>>> float(10)10.0

The previous examples call the str(), int(), and float() functions and pass them values of the other data types to obtain a string, integer, or floating-point form of those values.

The str() function is handy when you have an integer or float that you want to concatenate to a string. The int() function is also helpful if you have a number as a string value that you want to use in some mathematics.

For example, the input() function always returns a string, even if the user enters a number. Enter spam = input() into the interactive shell and enter 101 when it waits for your text.

>>> spam = input()101>>> spam'101'

The value stored inside spam isn’t the integer 101 but the string '101'. If you want to do math using the value in spam, use the int() function to get the integer form of spam and then store this as the new value in spam.

>>> spam = int(spam)>>> spam101

Now you should be able to treat the spam variable as an integer instead of a string.

>>> spam * 10 / 5202.0

Note that if you pass a value to int() that it cannot evaluate as an integer, Python will display an error message.

>>> int('99.99')Traceback (most recent call last):File "<pyshell#18>", line 1, in <module>int('99.99')ValueError: invalid literal for int() with base 10: '99.99'>>> int('twelve')Traceback (most recent call last):File "<pyshell#19>", line 1, in <module>int('twelve')ValueError: invalid literal for int() with base 10: 'twelve'

The int() function is also useful if you need to round a floating-point number down.

>>> int(7.7)7>>> int(7.7) + 18

In your program, you used the int() and str() functions in the last three lines to get a value of the appropriate data type for the code.

print('What is your age?') # ask for their agemyAge = input()print('You will be ' + str(int(myAge) + 1) + ' in a year.')

The myAge variable contains the value returned from input(). Because the input() function always returns a string (even if the user typed in a number), you can use the int(myAge) code to return an integer value of the string in myAge. This integer value is then added to 1 in the expression int(myAge) + 1.

The result of this addition is passed to the str() function: str(int(myAge) + 1). The string value returned is then concatenated with the strings 'You will be ' and ' in a year.' to evaluate to one large string value. This large string is finally passed to print() to be displayed on the screen.

Let’s say the user enters the string '4' for myAge. The string '4' is converted to an integer, so you can add one to it. The result is 5. The str() function converts the result back to a string, so you can concatenate it with the second string, 'in a year.', to create the final message. These evaluation steps would look something like Figure.

The evaluation steps, if 4 was stored in myAge

Summary

You can compute expressions with a calculator or type string concatenations with a word processor. You can even do string replication easily by copying and pasting text. But expressions, and their component values—operators, variables, and function calls—are the basic building blocks that make programs. Once you know how to handle these elements, you will be able to instruct Python to operate on large amounts of data for you.

It is good to remember the different types of operators (+, -, *, /, //, %, and ** for math operations, and + and * for string operations) and the three data types (integers, floating-point numbers, and strings) introduced in this chapter.

A few different functions were introduced as well. The print() and input() functions handle simple text output (to the screen) and input (from the keyboard).

The len() function takes a string and evaluates to an int of the number of characters in the string. The str(), int(), and float() functions will evaluate to the string, integer, or floating-point number form of the value they are passed.

In the next chapter, you will learn how to tell Python to make intelligent decisions about what code to run, what code to skip, and what code to repeat based on the values it has. This is known as flow control, and it allows you to write programs that make intelligent decisions.

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