Getting String Input
Before you can finally write “real programs,” you’ll need to be able to write scripts that can query the user for more information. Fortunately, Python includes a powerful built-in function, the input function, which makes this easy.
I’ll give you the syntax first and then show examples.
string_var = input(prompt_string)
The essence of this syntax is that the built-in input statement both takes and produces a text string. In one way, the concept of text string is easy to understand; it’s just “words” for the most part—or more accurately, letters and other characters.
For example, we might write a function, main, which we’re going to use as a script.
name1_str = input(’Enter your name: ’)
name2_str = input(’Enter another: ’)
name3_str = input(’And another: ’)
print(’Here are all the candidates: ’)
print(name1_str, name2_str, name3_str)
Enter your name: Brian
Enter another: Hillary
And another: Donald
Here are all the candidates:
Brian Hillary Donald
This by itself is not a very exciting program. It takes some text and displays it on the console. But this little program demonstrates an important ability of Python: the ability to prompt the user for a text string and then assign it to a variable.
While other variables up until now have referred to numeric values, these variables—name1, name2, and name3—all refer to text strings in this case.
What exactly can go into a text string? Basically, anything you can type can go in a text string. Here’s an example:
>>>in_str = input(’Enter input line: ’)
Enter input line: When I’m 64...
’When I’m 64...’
As you can see, text strings can contain numerals (digit characters). But until they’re converted, they’re just numerals. They are text-string representations of numbers, not numbers you can perform arithmetic on.
If this isn’t obvious, just remember that the numeral 5 is just a character on a keyboard or on the screen. But the number 5 can be doubled or tripled to produce 10 or 15 and has little to do with characters on a keyboard.
Here’s an example:
in_str = ’55’
But assigning 55 with no quote marks around it does something different.
n = 55
The difference is that 55 is an actual number, meaning that you can add, subtract, multiply, and divide it. But when enclosed in quotation marks, ’55’ is a text string. That means it is a string consisting of two numerals, each a 5, strung together.
A simple program should help illustrate the difference.
in_str = input(’Enter your age: ’)
print (’Next year you’ll be’, in_str + 1)
Enter your age: 29
Error! Incompatible types.
Oops! What happened? The characters 29 were entered at the prompt and stored as a text string, that is, a numeral 2 followed by a numeral 9—a string two characters long. But that’s not the same as a number, even though it looks like one.
Python complains as soon as you try to add a text string to a number.
in_str + 1
No error is reported until you execute the function. Python variables don’t have types; only data objects do. Consequently, Python syntax seems lax at first. But the types of data objects—which are not checked for syntax errors, as there are no “declarations”—are checked whenever a Python statement is actually executed.
This means, among other things, that you cannot perform arithmetic on a string of numerals such as 100, until that string is first converted to numeric format. If this doesn’t make sense now, don’t worry; it will make sense when you read the next section.
The next section shows how to get input and store it as a number rather than text string.