Your Basic Python

Part of JU2602
Mikko Hellsing, mikko@sumsum.se

What we will talk about today

Basic data types: strings, numbers
Variables -What are they and how to use them?
More advanced data types: lists, dictionaries
Control structures: if, for
Functions: def
If there is time: modules and example programs

How to write strings

>>> print('Hello multiple worlds!')
Hello multiple worlds!

>>> 'Hello' == "Hello"
True

How to index strings

>>> 'Karl Popper'[1]
'a'

>>> 'Guido Van Rossum'[0:3]
'Gui'

>>> 'Guido Van Rossum'[:3]
'Gui'

>>> 'Guido Van Rossum'[14:]
'um'

>>> 'Guido Van Rossum'[-2:]
'um'

String ranges

Concatenate strings

>>> '{} {}'.format('one', 'two')
'one two'

>>> '{0} {1} {0} {1} and so on...'.format('one', 'two')
'one two one two and so on...'

>>> 'a' + 'b' + 'c' + 'd' # You *can* do this too but...
'abcd'

Transform strings

>>> 'abc'.upper()
'ABC'

>>> 'XYZ'.lower()
'xyz'

>>> '  Interesting text      '.strip()
'Interesting text'

Calculate with numbers

>>> 1 + 1
2

>>> 3.14 * 100
314.0

>>> 30 / 10
3.0 

>>> 0.1 + 0.2
# internal representation of floats is not exact
0.30000000000000004

All together now

>>> 'JU' + 2602
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: must be str, not int

>>> 'JU{}'.format(2602)
JU2602

Variables

Some naming rules/conventions

Use lower case letters and underscore
Be descriptive but not overly verbose
Don't start with a number, infact avoid using numbers
Using one letter variable names violates #2

Examples

drink = 'The green elevator'
first_name = 'Greta'
participants = 5034132
cards = [1, 2, 3, 4, 5, 6, 7, 8 ,9 ,10, 'J', 'Q', 'K', 'A']
name_pat = re.complie('(w+) (w+)')
person = Human(conciousness=False)

Lists

Accessing elements

>>> grades = ['Underkänt', 'Godkänt', 'Väl Godkänt']
>>> grades[1]
'Godkänt'

>>> grades[-2]
'Godkänt'

>>> grades[0]
'Underkänt'

Append, pop & len

>>> currencies = ['SEK', 'NOK', 'DKK', 'EUR']
>>> currencies.append('PKK')
>>> currencies
['SEK', 'NOK', 'DKK', 'EUR', 'PKK']

# But PKK is not a currency
>>> currencies.pop()
'EUR'
>>> len(currencies)
4
>>> currencies
['SEK', 'NOK', 'DKK', 'EUR']

Extend, index & sort

>>> currencies.extend(['GBP', 'BTC'])
>>> currencies
['SEK', 'NOK', 'DKK', 'EUR', 'GBP', 'BTC']

>>> currencies[-2]
'GBP'

>>> currencies.remove('BTC')
>>> currencies
['SEK', 'NOK', 'DKK', 'EUR', 'GBP']

>>> currencies.sort()
>>> currencies
['DKK', 'EUR', 'GBP', 'NOK', 'SEK']

List iteration

>>> for cur in currencies:
>>>     print(cur)
DKK
EUR
GBP
NOK
SEK

>>> for i in [0, 1, 2]:
>>>     print(i, 'is less than', i + 1)
0 is less than 1
1 is less than 2
2 is less than 3

All together now

>>> trix = ['tailslide', 'boardslide', 'grind', 'boneless']
>>> 'ZERO {} TWO'.format(trix[-1][1:4].upper())
'ZERO ONE TWO'

>>> primes = [2, 11, 7, 13, 5]
>>> primes.append(3)
>>> primes.sort()
>>>
>>> for p in primes[2:-2]:
>>>     print(p)
5
7

One more thing...

>>> times_two = [x * 2 for x in [1, 2, 3]]
>>> times_two
[2, 4, 6]

# Using Python 3.6+ f-string
>>> [f'{c}ello' for c in ['H', 'Y', 'M']]
['Hello', 'Yello', 'Mello']

>>> [x * x for x in [1, 2, 3, 4, 5, 6, 7]]
[1, 4, 9, 16, 25, 36, 49]

>>> [x * x for x in range(1, 8)]
[1, 4, 9, 16, 25, 36, 49]

Tuples vs Lists

A tuple acts alot like a list but it is immutable, that means that it's contents cannot be changed, it can be redefined but not altered.

>>> salaries = [45000, 28000]
>>> salaries.append(33000)  # salaries = [45000, 28000, 33000]

>>> salaries = (45000, 28000)
>>> salaries.append(33000) # Not allowed, will generate error.

>>> [s * 12  for s in salaries]
[540000, 336000]

Let's try it out

Programming using if should match your intuition pretty well.

>>> if True:
>>>     print('Amazing')
Amazing

>>> if len('Lionel') == len('Hello'):
>>>     print('It is not me you are looking for.')
>>> else:
>>>     print('Is it me you are looking for?')
Is it me you are looking for?

`If elif else`

>>> if 1 == 3:
>>>     print('Wow!')
>>> elif 1 == 2:
>>>     print('Wow!')
>>> else:
>>>     print('Boring.')
Boring.

>>> if 1 != 3 and 2019 > 2018:
>>>     print('You enlighten me!')
You enlighten me!

Nested `if` & lists

>>> stuff = ['Paper', 'Pen', 'Eraser', 'TV']
>>> if 'Laptop' in stuff:
>>>     print('You will be successful!')
>>> else:
>>>     if 'Pen' in stuff and 'Paper' in stuff:
>>>         print('Evolution, ever heard of that?')
>>>     else:
>>>         print("It's not like you are the future but ok.")
>>>     print('You are bound for unsuccess!')
Evolution, ever heard of that?
You are bound for unsuccess!

Dictionaries

>>> colours = {'C': 'Cyan', 'M': 'Magenta',
'Y': 'Yellow', 'K': 'Key'}
>>> colours['Y']
'Yellow'

>>> colours['R'] = 'Red'
>>> colours.update({'G': 'Green', 'B': 'Blue'})
>>> colours
{'C': 'Cyan', 'M': 'Magenta', 'Y': 'Yellow',
'K': 'Key', 'R': 'Red', 'G': 'Green', 'B': 'Blue'}

Popping and iterating

>>> ages = {'pope': 82, 'trump': 72, 'merkel': 65}
>>> ages.pop('trump')
72
>>> for name, age in ages.items():
>>>     print(f'{name}: {age}')
pope: 82
merkel: 65

Keys n' values

>>> food = {'p': 'Potato', 't': 'Tomato'}
>>> food.keys()
dict_keys(['p', 't'])

>>> for k in food.keys():
>>>     print(k)
p
t

>>> for v in food.values():
>>>     print(v)
Potato
Tomato

All together now!

>>> junk_food = {
>>>   'burgers': ['normal', 'cheese', 'veggie'],
>>>   'fries': ['normal', 'large'],
>>>   'drinks': ['soft', 'milkshake'],
>>> }

>>> for btype in junk_food['burgers']:
>>>     print(f'Who wants a {btype} burger?')
Who wants a normal burger?
Who wants a cheese burger?
Who wants a veggie burger?

Functions

Define a function with `def`

A function is some process that you can run by calling the function name.

>>> def hello():
>>>     print('Greatings earthling!')
>>>
>>> hello()
Greatings earthling!

>>> def hello_there(name):
>>>     print(f'Hello there {name}!, long time no see!')
>>>
>>> hello_there('Bobby')
Hello there Bobby!, long time no see!

Default arguments

>>> def diagnose(name, disease='a cold'):
>>>     """Rigorous diagnostic procedure is performed here"""
>>>     print(f'{name} has {disease}')
>>> 
>>> diagnose('Fardosa')
Fardosa has a cold

>>> diagnose('Bob', 'N.P.D.')
Bob has N.P.D.

Returning values

>>> def is_even(x):
>>>     """Checks if a number is even"""
>>>     return x % 2 == 0
>>>     
>>> for x in range(6):
>>>     if is_even(x):
>>>         print(f'{x} is an even number')
0 is an even number
2 is an even number
4 is an even number

Some good ideas on functions

Make each function have one responsability
Don't write long functions, keep them under ~50loc
Include a docstring
Don't nest too many ifs, keep it simple and readable
Use the same naming rules as for variables

What is a module?

A module is just a file containing python code.

Example module `cat`

The file cat.py is a module. It imports a module: random, defines 2 functions: cat_name and cat_life and defines one constant: CATIQ.

import random

CATIQ = 34 

def cat_name():
    return random.choice(['Maja', 'Sigge', 'Selma', 'Nisse'])

def cat_life():
    return random.normalvariate(10, 5)

Module with main program

Example squares.py:

def nsq(n):
    return [i ** 2 for i in range(1, n + 1)]


if __name__ == '__main__':
     print(nsq(10))

Your Basic Python

What we will talk about today

How to write strings

How to index strings

String ranges

Concatenate strings

Transform strings

Calculate with numbers

All together now

Variables

Some naming rules/conventions

Examples

Lists

Accessing elements

Append, pop & len

Extend, index & sort

List iteration

All together now

One more thing...

Tuples vs Lists

Let's try it out

If elif else

Nested if & lists

Dictionaries

Popping and iterating

Keys n' values

All together now!

Functions

Define a function with def

Default arguments

Returning values

Some good ideas on functions

What is a module?

Example module cat

Module with main program

`If elif else`

Nested `if` & lists

Define a function with `def`

Example module `cat`