Talking to the User: Quick Revision

A brief note on Talking to the User. Read it before a test. Try each question at the end before you open its answer.

Asking with input()

• input("question") shows the question, waits for an answer and gives the answer back. Keep it in a variable: city = input("Your city? ").
• On this site, type the answers in the Input box before you press Run, one on each line. Each input() reads the next line.
• Too few lines in the Input box give an EOFError.
• input() always gives text, even when the user types digits.
• int() makes a whole number, float() a decimal number and str() text. int(input("How many? ")) asks and converts in one line.
• Plan a small program in three parts: input, calculation, output. Give the answer a label and a unit, and round it.

Strings: positions and slices

• A string is a row of characters. len() counts them, spaces too.
• The positions start from 0. A negative position counts from the end, and -1 is the last character.
• A position that is too big gives an IndexError.
• word[start:stop] takes the characters from start up to, but not including, stop.

With word = "GARDEN":

• A string cannot be changed in place. Make a new string instead.

String methods and f-strings

• upper(), lower(), title(), strip() and replace() give a new string. Store it if you want to keep it: name = name.strip().
• in, startswith(), endswith(), isdigit() and isalpha() answer True or False.
• count() counts a piece. find() gives the position of the first match, or -1 when it is not there.
• + joins strings with no space between them. * repeats a string: "=" * 10.
• An f-string puts values inside text: f"{name} scored {marks}". No str() is needed.
• {x:.2f} shows two decimal places. {x:>8} puts the value on the right of 8 spaces, and {x:<8} on the left.

Python on your own computer

• You do not need to install anything for this book. Install Python to keep your programs as files and to work offline.
• You need three tools: Python, an editor and a terminal. Download Python only from python.org.
• On Windows, tick Add python.exe to PATH. Check the installation with py --version, or python3 --version on a Mac.
• A program is a text file that ends in .py. Run it in IDLE with F5, or in a terminal with python3 hello.py from the folder of the file.
• cd goes into a folder. python -m pip install name adds a package.

A worked example

Dev wants a program that asks for the price of one notebook and the number of notebooks, then shows the bill.

1. Plan in words. Input: the price of one notebook in rupees, and the number of notebooks. Calculation: price times number. Output: the bill, with a label and a unit.
2. Read and convert at once. input() gives text, so the price goes through float() and the number through int().
3. Calculate. bill = price * count.
4. Show it well. An f-string with :.2f gives the bill in rupees with two decimal places.
5. Test with numbers you know. For 24.5 rupees and 4 notebooks, the bill must be 98.00 rupees.

Without float() and int(), the third line would try to multiply two pieces of text. Python stops with TypeError: can't multiply sequence by non-int of type 'str'.

Practice with new numbers

A program to try

The program makes a username from the first four letters of a first name and the last two digits of a year.

Mistakes to avoid

• Forgetting that input gives text. input("Age? ") + 1 stops with TypeError: can only concatenate str (not "int") to str. Convert first: int(input("Age? ")).
• int() on a decimal. int("12.5") stops with ValueError: invalid literal for int() with base 10: '12.5'. Use float() for a decimal number.
• Counting from 1. The first character is word[0]. In a word of 3 letters, word[3] stops with IndexError: string index out of range.
• Not storing a method's answer. name.upper() on its own line changes nothing. Write name = name.upper().
• Changing one letter. word[0] = "B" stops with TypeError: 'str' object does not support item assignment. Build a new string: "B" + word[1:].
• A file called hello.py.txt, or random.py. The first does not run as a program. The second hides the module of the same name.

Check yourself

1. Priya writes age = input("Age? ") and then print(age + 5). What happens, and how should she change the first line?

2. With word = "LANTERN", what are word[2], word[-2] and word[2:5]?

3. A sports club keeps the fee per month in fee = 37.5. Which f-string shows it as Fee: 37.50 rupees?

4. What do "banana".find("n") and "banana".count("a") give?

5. Your program marks.py sits in a folder called python_work, but the terminal is open in another folder. Which two steps run the program?