Random Live Hacking

Data Types

• Variables are just names that point to objects

• Variables are not typed - the type is in the objects they points to

i = 1

type(i)

int

i = 1.5

type(i)

float

i = [1,2,3]

type(i)

list

int is also a variable - a name. It points to an object containing the type for all int objects.

print(int)

<class 'int'>

What is the type of that int type object? What’s the type of type objects, generally?

print(type(int))

<class 'type'>

Aha …

int('42')

42

int is just a name, so let another name point to the same object (type int)

franziska = int

franziska('42')

42

Assignments to int are possible. Although rarely useful.

int = 42

try:
    int('42')
except BaseException as e:
    print(e)

'int' object is not callable

Remove the last name for type int, so it is gone forever …

del franziska

Ah, it is still there: 1 is an integer literal, so it has to carry a reference to its type. This comes to our rescue: we can restore the name int to point to what is should.

type(1)

int

int = type(1)

int('42')

42

Pooh!

Mutable, Immutable

l1 = [1, 2, 3]
l2 = l1
print(hex(id(l1)))
print(hex(id(l2)))

0x7f03d41165f0
0x7f03d41165f0

l1.append(4)
l1

[1, 2, 3, 4]

l2

[1, 2, 3, 4]

Exception, demonstrated using dict access

d = {1:'one', 2:'two'}

d

{1: 'one', 2: 'two'}

d[1]

'one'

d[2]

'two'

Here we access a nonexisting dictionary member, demonstrating how we react on the error that ensues. * Catch the exception by type (KeyError) * Use the logging module to format the output (stack trace etc.). See its docs for more.

import sys
import logging

try:
    d[3]
except KeyError as e:
    print('Jessas:', e)
    logging.exception('verdammt!')

ERROR:root:verdammt!
Traceback (most recent call last):
  File "<ipython-input-25-bd3a1af9b474>", line 5, in <module>
    d[3]
KeyError: 3

Jessas: 3

print(type(KeyError))

<class 'type'>

All Exceptions are derived from BaseException

issubclass(KeyError, BaseException)

True

Even the ones that you define yourself have to be derived from BaseException. Better yet, derive them from Exception which should the base for all user-defined exceptions.

try:
    raise 'bummer!'
except BaseException as e:
    print('Cannot raise str:', e)

Cannot raise str: exceptions must derive from BaseException

Indices and Slicing

l = ['Peter', 'Paul', 'Mary']

peter = l[0]
peter

'Peter'

peter[0:3]

'Pet'

l[0][0:3]

'Pet'

peter[:3]

'Pet'

l = [2,3,4]
l[0:0]

[]

l[0:0] = [0,1]
l

[0, 1, 2, 3, 4]

for loops

Lists are perfectly iterable

for item in ['blah', 'bloh', 'blech']:
    print(item)

blah
bloh
blech

range is not a list, but a generator. A list would contain (allocate in memory) all that it has. range produces the next element on demand - as the for loop iterates.

for i in range(10):
    print(i)

0
1
2
3
4
5
6
7
8
9

for i in range(5,10):
    print(i)

5
6
7
8
9

range(10)

range(0, 10)

list(range(10))

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

r = range(10)

Iterator protocol

iterator = iter(r)
iterator

<range_iterator at 0x7f03d402e570>

next(iterator)
next(iterator)
next(iterator)
next(iterator)
next(iterator)
next(iterator)
next(iterator)
next(iterator)
next(iterator)

8

next(iterator)

9

# final element already consumed, consume another one
try:
    next(iterator)
except StopIteration as e:
    print(e)