Getting Started with Python: Quick Revision

A brief note on Getting Started with Python. Read it before a test. Try each question at the end before you open its answer.

Programs and print()

• A program is a list of instructions. Python does them from the top to the bottom, exactly as written.
• print() shows things on the screen. Text goes inside quotation marks: print("Hello").
• Without quotation marks, Python works out the sum first: print(3 + 4) shows 7.
• Pieces separated by commas get one space between them. sep changes the space, and end changes what comes after.
• A line that starts with # is a comment. Python skips it; it is there for the person who reads the program.

When something goes wrong

• Read an error message from the bottom. The last line names the kind of error; the line above shows where.
• A SyntaxError means Python cannot read the line, often a missing bracket or quotation mark.
• A NameError means a name Python does not know: a spelling mistake, a wrong capital letter, or a variable used before it was made.
• Fix one thing at a time, and run again.

Variables and the four types

• A variable is a name for a value: marks = 72. The value on the right goes into the name on the left.
• A new value replaces the old one. score = score + 10 works out the right side first. += is the short form.
• A variable holds a value, not a rule. Changing one variable later does not change another.

• type() tells you the type. int(), float() and str() change a value from one type to another.
• + adds numbers but joins text, so "4" + "5" is "45". You cannot join text and a number.
• A good name has small letters, digits and underscores, does not start with a digit, and says what it holds: total_marks.

Python as a calculator

• The order is the order of mathematics: brackets, then powers, then * / // %, then + -. When you are not sure, use brackets.
• quotient * divisor + remainder gives back the number you started with.
• round(x, 2) rounds to 2 places. int(x) cuts the decimal part off. abs(), min(), max() and divmod() are ready to use.

A worked example

What does this program show?

Follow it one line at a time, with a trace table.

1. Line 2 uses the value of x at that moment, 7, so y is 14.
2. Line 3 changes x to 8. The value of y does not change: it holds 14, not the rule "twice x".
3. Line 4 works out 14 - 8, so y becomes 6.

The program shows 8 6.

Practice with new numbers

A program to try

A farm packs its eggs into boxes of 12. The program finds the full boxes, the eggs left over and the money for the full boxes.

Mistakes to avoid

• A capital letter. Print("Hi") stops with NameError: name 'Print' is not defined. Python reads print and Print as different names.
• Joining text and a number. print("Age: " + 12) stops with TypeError: can only concatenate str (not "int") to str. Use a comma, or str(12).
• / for a whole number. / always gives a float: 8 / 4 is 2.0. Use // for a whole number of times.
• int() for rounding. int(4.9) is 4, because it cuts off. round(4.9) is 5.
• Naming a variable print or sum. The name now holds your value, and the tool stops working.

Check yourself

1. Ravi shares 23 marbles among 4 friends, the same number each. Which two Python sums give each friend's share and the marbles left with Ravi, and what are they?

2. A program has price = 40, then total = price * 3, then price = 50. What is total at the end?

3. Which of these can be a variable name: 2nd_place, total_marks, my-score, class?

4. Name the type of each of these four values: 7, 7.0, "7" and 7 > 3.

5. Meena works out -3 ** 2 in Python and gets -9, but her calculator gives 9 for minus 3 squared. Why, and how should she write it?