الدرس 02 · Basics

Numbers and strings

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Every value in Python has a type. Use the >>> prompt as a calculator and meet the three ways to divide, then step through a five-line program, tea.py, that makes an int, a float and a string, prints them, and turns a number into text with str(). Finish with what happens when you mix text and a number without converting: a TypeError.

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01What does Python give for 7 / 2 ?

/ always gives a float, so 7 / 2 is 3.5. Use // to drop the fraction (3) and % for the remainder (1).

02What is 2 + 3 * 4 ?

As in maths, * is worked out before +: 3 * 4 is 12, plus 2 is 14. Write (2 + 3) * 4 to get 20.

03What is the type of 5 * 1.5 ?

1.5 is a float, and an int combined with a float gives a float: 7.5.

04What does "ha" * 3 give?

* repeats a string: "ha" * 3 is 'hahaha'. + would join two strings end to end.

05cups is the int 5. Which line prints Cups: 5 without an error?

+ joins a string only to another string. str(cups) turns 5 into '5'. Without it Python stops with TypeError: can only concatenate str (not "int") to str.

المصطلحات الأساسية

المسرد

Type: The kind of a value, such as int, float or str. It decides what you can do with the value; type() tells you a value's type.
int: A whole number such as 5 or -12. Python ints can be as big as you like.
float: A number with a decimal point, such as 1.5 or 3.0. Stored in binary, so it can be very slightly off (0.1 + 0.2).
str (string): Text in single or double quotes, such as "tea". + joins strings and * repeats them.
Operator: A symbol that works on values: + - * / // % ** for numbers, + and * for strings.
Floor division and remainder: // divides and rounds down (7 // 2 is 3); % gives what is left over (7 % 2 is 1).
Conversion: Making a value of one type from another: str(5) is '5', int("42") is 42, float("1.5") is 1.5.
TypeError: The error Python raises when an operation gets a value of the wrong type, such as joining a str and an int with +.

تفاصيل إضافية

الملاحظات والمصادر

The $ and >>> at the start of a line are prompts printed for you: you do not type them. On macOS and Linux the command may be python3.
A float is stored in binary with about 15 to 17 significant digits, so most decimal fractions such as 0.1 are stored very slightly off; round(0.1 + 0.2, 2) gives 0.3.
// rounds down, toward minus infinity, so -7 // 2 is -4; and % follows it, so -7 % 2 is 1.
Python 3.11 and later also print a line of ~ and ^ under the failing code, pointing at the +; the figure leaves it out. In the real message, the ... is the full path to the file.
Python ints have no fixed size: 2 ** 100 is exact. Floats do have limits; the largest is about 1.8 × 10 to the power 308.

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