Skip to content

Latest commit

 

History

History
219 lines (189 loc) · 11.9 KB

02_variables.md

File metadata and controls

219 lines (189 loc) · 11.9 KB
__     __         _       _     _           
\ \   / /_ _ _ __(_) __ _| |__ | | ___  ___ 
 \ \ / / _` | '__| |/ _` | '_ \| |/ _ \/ __|
  \ V / (_| | |  | | (_| | |_) | |  __/\__ \
   \_/ \__,_|_|  |_|\__,_|_.__/|_|\___||___/

Naming Variables

Variable names follow snake_case
Constants are in SCREAMING_SNAKE_CASE
Privates have a _leading_underscore

WARNING
The Python interpreter could not care less about what's private or constant and will glady let you import a private variable from wherever and change a constant. This naming convention is just for IDEs and devs, so the whole thing operates on scouts honour.

Quick Note on the Typing System

Python is dynamically typed and strongly typed. So, variables can change types willy-nilly (dynamic) but using an incompatible type will throw an error (strong).
We don't have to declare types, but we can do type hinting though - again - the interpreter doesn't give a damn about type hints, they're just for IDEs and devs. We'll get to type hints later since they could be their own entire lesson.

Booleans

You've got bool.
Declare it by assigning either True or False:

my_bool = True

No value

You've got None, in other languages you'll have seen null or void. It's what you get when there ain't nothing to give.
Declare it by assigning None:

my_none_type = None

Numbers

Whole numbers

You've got int, no long, no short, no int32.
Declare it by assigning a number without a decimal place:

my_int = 1

If you've got a real big number, you can use an underscore as a thousands separator.
e.g.

my_big_int = 1_000_000

The upper bound of integers is limited by memory, they are theoretically unbound. So you don't have to worry about overflow issues here.
Example:

Python 3.12.4 (main, Jun  6 2024, 18:26:44) [GCC 11.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 2**9000
1861919823602446870051646947443265806218680881620164150161309755333181392347475969737916885903925979688282376725245050928552019850699830936678289110574564545897808148390669479768088103164174862614314896361751492419355200744919106773071079582599147802839706215610874713577461171484389014531465527630202236883843662826326484204605969450292309212479576754101484210438029392847117393209646612418243468945316443055403112189769994194382253211069075682210324665380367163398351260390065146550963353638226230571087153090476005831034463871223599543367440127371321307935600294235235555686651194174520755709233252147269712646152398661272615394330256766919854213808655136454478279520538925315159120689449046824765276993871084725126289174642261134186532952389529497451225294280309592333118547431857834022107299493307642886550035108349812538868548204110949267007844894921304724439232280030484032946555862021618822463975371563390466476093942333021906014621095132521668771544547398222309222385454252810109269995041305909748903485123565191403462125774254093965366838199092942669763234497755795922782368415391408328615941087687168142703569478330639528167193433105543630476588701348505543567751790789117717660152741734303989082897740369036292716627460893145378475903236426283709902853825833522685221030263417298462852391838147572583018386475574242223720125609852369550108535609838296845885540331405153336504029657425852682616998765068978970510668576704757343969900471616618487531217423608126248306049345412199702325217046285213249285846153975313978242675003402962824121160875928608618335006217051712260195130737083017631349816669929159176667868417353866905513835554999462637481342721400975639504482716709384155856026263304270260886740810287158032302083388891446551907198669473320114371718138443660594911466234734942910722449045244457975320043694169261701681656148362205170532994281152576853709669767416127252738437485793910219846836498268031157261792859230067295449184914230926461991700842371010236588596679342492743911615365711320743215987995901101163306076544568626531198809370907612331483471833845958618077480217892890533116603350040147116218227928732817945323636422047507156427135297222275812678187736036773320854938458989641651199078792658709405149957007882875374868180442468709128120647047303125901843060904208622152998500617510802742432155144605848971282208131749602408191808406410632175265171306237793956570406435402345476628467209068729893134433124291187516075267594298323098487990166243403468912628117388443154921474663149036640983804471509564893426039130480070632369458141251485052936042164865777219873983089572387511794739572152294019631222145910901210840561445507713655658889336373566800391206925212386456974315749376

Decimals

You've got float, no doubles, no float64.
Declare it by assigning a number with a decimal place:

my_float = 1.0

Floats are limited by your system architecture, you'll get 32bit floats on a 32bit system etc. So please do feel free to worry about this one.

Text

There's just strings, there's no chars.
Declare it by assigning something between quotes:

my_str = "Hello World!"
my_other_str = 'Hello Galaxy!'
# The following two can start and end on different lines
# and will include any line feeds you put in them
my_str_which_is_usually_reserved_for_doc_strings = """Hello Universe!"""
my_str_which_is_quite_uncommon = '''Hello Multiverse!'''

We'll see docstrings later in this lesson, so don't fret if you didn't understand that bit

need command note
access character at index my_str[index] cannot be used to change value
discern if substring in str substring in my_str returns either True or False AND substring can be any length including 0 or 1
concatenate 2 strings str1 + str2 returns new string
str1 += str2 modifies str1
'some text ' 'some more text' returns new string - very useful for abiding by the line length PEP8 standard (see example below)

So, at some point you will want to use a very long string but PEP8 will shout at you for it:

my_string = "The quick brown fox jumps over the lazy dog, passes to the snowman and goooooaaaallll!"

PEP8 flags that up because it's too bloody long. The solution, wrap it up in a set of parentheses and then do that last concatenation technique:

my_string = (
    "The quick brown fox jumps over the lazy dog,"
    " passes to the snowman and goooooaaaallll!"
)

Smashing!

Containers

QUICK NOTE: Any of the objects under this section (and strings as it happens) can have their length checked by passing them into the len() function.
SECOND QUICK NOTE: These objects can't be copied with my_copy = your_copy, it has to be my_copy = your_copy.copy() but this is only a shallow copy, deep copies will come in a later lesson.

Key-Value Pairs (dict)

We call them dictionaries, you might call them dictionaries, hash-maps, hash-tables, or something like that.
Declare it by assigning some comma separated key-value pairs between braces, a key must be separated from a value by a colon :.

my_dict = {'key': 'value', 'a': 1, 1: 'one'}  # filled
my_dict = {}  # empty
need command note
access value my_dict[key] throws error when key doesn't exist
my_dict.get(key) returns None if key doesn't exist OR if key does exist but value is None
change value OR assign new key-pair my_dict[key] = value
merge dictionaries dict1 | dict2 returns new dict AND if key exists in both dicts then value from dict2 takes priority N.B. only available in Python3.9 or higher
{**dict1, **dict2} returns new dict AND same key priority as above AND available regardless of version
dict1.update(dict2) returns None just updates dict1 AND same key priority as above AND available regardless of version
remove key-pair del my_dict[key]
list keys my_dict.keys() does not return actual list object but mostly acts the same
list values my_dict.values() does not return actual list object but mostly acts the same
discern if key in dict key in my_dict returns either True or False

Additional notes

Dictionaries don't have a fixed length, and can contain any mix of types for both keys and values.
Keys must be unique and hashable, to check if something is hashable use a CLI interpreter to pop it into the hash() function (e.g. hash('HASH ME!') ) as unhashable types throw errors.

Value only, duplicates allowed (list)

There's only list, no arrays, no vectors.
Declare it by assigning some comma separated values between square brackets:

my_list = [1, 2, 'c', 'd']  # filled
my_list = []  # empty
need command note
access index my_list[index] can be negatively indexed to access values from other end (i.e. my_list[-1] returns last value, my_list[-2] returns penultimate value, etc)
add to list my_list.append(value)
merge lists list1 + list2 returns new list
[*list1, *list2] returns new list
list1.extend(list2) modifies list1
remove index del my_list[index]
remove value my_list.remove(value) Only removes first instance of value AND throws error if value not present
sort list my_list.sort() sorts in place
reverse list my_list.reverse() reverses in place
discern if value in list value in my_list returns either True or False

Additional notes

Lists don't have a fixed length, and can contain any mix of types.

Value only, no duplicates (set)

We call this a set.
Declare it by assigning some comma separated values between braces:

my_set = {1, 2, 'c', 'd'}  # filled
my_set = set()  # empty, remember {} is empty dict
need command note
add value my_set.add(value)
remove value my_set.discard(value) doesn't throw error if value not present
discern if value in set value in my_set Returns either True or False

Additional notes

Sets don't have a fixed length, and can contain a mix of types (so long as they're hashable but we'll get to that later).
Sets don't maintain order, so you can't access a specific element.

Truthy/Falsy

There are truthy/falsy values in Python. Everything is truthy unless it's; zero, None, False, or empty.

There is a really cool thing you can do in Python with Truthy/Falsy assignment. You can chain variables with the or keyword and the first one to not be Falsy is what will be assigned. e.g.

my_list = []
my_dict = {}
my_int = 42
my_float = 3.9

something_truthy = my_list or my_dict or my_int or my_float

something_truthy will take on the value of 42 as everything before it was Falsy.

Declaring Types!

So this is another thing the interpreter doesn't care about. Declare it one way and assign it the other, interpreter don't mind. A well set up IDE will help you keep to the declared types but really this is for you and the other devs working on your code.

You've already seen all the types because I formatted them as inline code to cement it in your brain before you got here.
So it goes like this:

my_int: int = 42
my_list: list = [1, 2, 3, 4]
# so on and so forth

It just goes name: type = value
If you want to declare the content of a container, you get:

my_list: list[str] = ['this', 'is', 'cool']
my_dict: dict[int, str] = {1: 'one', 2: 'two'}
# N.B. You can't do it this way in Python3.8 but that's deprecated in two months anyway (Oct 2024)

Say you want to state a variable can take one value or another, it depends on the runtime version.
Pre 3.10:

from typing import Union

my_list = list[Union[int, str]] = [5, 4, 3, 2, 1, 'blast off!']

3.10 and beyond:

my_list = list[int | str] = [5, 4, 3, 2, 1, 'blast off!']

That's it?

Yeah, for now.

Moving on!