Getting Started with Python

Python’s identifiers and keywords, data types, operators and their precedence, type conversion, input and print, and syntax, runtime and logical errors.

Key ideas

An identifier starts with a letter or _, has only letters, digits and _, is case-sensitive, and cannot be a keyword such as for, True or None.
The basic types are int, float, complex, bool, str, list, tuple, dict and NoneType; type(x) tells you which.
/ always gives a float; // rounds the quotient down (towards minus infinity), and % gives a remainder with the sign of the divisor.
** is done first and groups from the right, even before a unary minus; then *, /, // and %; then + and −; then comparisons; then not, and, or.
input() always gives a string; int() and float() convert it, and int() simply drops the fraction part.
and gives its first false value (else the last), and or its first true value (else the last): not always True or False.
A syntax error stops Python before the program runs, a runtime error stops it while running, and a logical error gives a wrong answer.

Remember

// and %	17 // 5 = 3, 17 % 5 = 2; −17 // 5 = −4, −17 % 5 = 3
They always fit	a = b × (a // b) + a % b
Precedence	5 + 3 * 2 = 11; 2 3 2 = 512; −3 ** 2 = −9
Floats	10 / 5 = 2.0; 7 // 2.0 = 3.0; 4 + 1.0 = 5.0; True + 4 = 5
round()	a tie goes to the even neighbour: round(2.5) = 2, round(3.5) = 4
Conversion	int('12') + 3 = 15; str(4) + str(2) = '42'; int(7.9) = 7
and, or	5 and 9 gives 9; 0 and 9 gives 0; 5 or 9 gives 5; 0 or 9 gives 9
print()	print(1, 2, sep='-') prints 1-2; end='' stops the new line

Watch out

Thinking // rounds towards zero: −7 // 2 is −4, not −3.
Forgetting that input() gives text: typing 5 for a = input() makes a * 2 print 55.
Writing 5 = x: the left side of = must be a name.
Expecting round(4.5) to be 5: it is 4.

Worked example

What does this program print? Give (a) the first line (b) the second.
a = 23
b = 1 - 23
print(a // b)
print(a % b)

Working: a = 23, b = −22: // rounds down (towards minus infinity), and a = b × (a // b) + a % b

Answer: (a) −2; (b) −21