Working with text

So far your programs have always printed the same thing. It gets interesting when a program asks a question and works with the answer. Python has the function input for this, and because an answer always arrives as text, this lesson also shows you how to put text together, tidy it up and turn it into numbers.

Asking for input

input prints a text, waits until someone types something and presses Enter, and returns what was typed. Usually you store it in a variable straight away:

name = input("What is your name? ")
print("Hello,", name)

When you run the program, an input field appears below the code. The space at the end of "What is your name? " keeps your answer from sticking to the question mark.

Putting text together

You join two strings with +, and * repeats one:

first = "Ada"
last = "Lovelace"
print(first + " " + last)
print("=" * 20)

As soon as several parts and variables come together, + quickly gets hard to read. f-strings are more convenient: you put an f in front of the first quotation mark and write variables in curly brackets right inside the text.

name = input("What is your name? ")
city = input("Where do you live? ")
print(f"Hello {name}, greetings to {city}!")

The brackets may even contain a small calculation, such as f"{2 * 21} is the answer".

Tidying up text

Strings come with functions of their own, called methods. You call them with a dot directly on the text:

answer = "   ada LOVELACE  "
print(answer.strip())
print(answer.strip().lower())
print(answer.strip().upper())
print(answer.strip().title())
print(answer.replace("ada", "Augusta Ada"))

strip removes spaces at the start and end, lower and upper make everything lower or upper case, title capitalises every word, and replace swaps one part of the text for another. None of these methods changes the original string; each returns a new one, which is why they can be chained as in the example. This is especially worth it with input, because people like to type one space too many or everything in lower case.

The function len tells you how long a text is:

word = "Informatics"
print(len(word))

From text to number

input always returns a string, even when someone types a number. Python cannot calculate with this text:

age = input("How old are you? ")
print(age + 1)

The solution is to convert the text first. int(...) turns text into a whole number, float(...) into a decimal number, and str(...) turns a number back into text:

age = int(input("How old are you? "))
print(f"Next year you will be {age + 1}.")

One function call sits inside the other here: Python first runs the inner input and then passes its result on to int. If someone types something that is not a whole number, such as twelve or 12.5, the program stops with a ValueError. How to deal with that without the program crashing comes in a later part.

Takeaways

• input("Question ") waits for input and always returns it as a string.
• f-strings like f"Hello {name}" put values right into a text.
• Methods such as strip, lower, upper, title and replace return a new, changed string; len counts the characters.
• int(...), float(...) and str(...) convert between text and numbers.