Introduction
First of all what is python? Python is a interpreted programming language,
it lets us talk to computers and ask them to do things for us.
We write python in plain text and pass it into the python interperter, it interperts the code
we give it into instructions the computer understands and will run those.
Sidenote: Python gets compiled into python bytecode which similar to java runs on a VM, This Vm gives the code a lot of extra features such as dynamic typing and error handling and portability of code but obviously comes with major speed hits. Where a language like C compiles directly to machine code that runs on your cpu, python is run on a layer of code that is constantly checking things and managing the enviornment. This is a tradeoff, you get simpler code that is easier to reason about but the speed tanks. That’s ok for most reasons, computers are quite fast and you are happy to wait a couple of seconds for results where doing them by hand would take months.
To talk to the python interperter you have to write code in the python syntax,
which is designed to be somewhat easy to pick up.
To play with python you can launch up a interpert session by opening a terminal
in your pycharm window and typing in python
Python 2 vs Python 3
Python 2 was the first verion of python that begun to be used accross the board, when python 3 came along there was so much code already written in python 2 and so many parts of it that would break with a update to python 3 that python 3 became a new programming language in it’s own right. The two languages are very similar but nto interchangeable. For your purposes use python3, it’s what everyone is aiming to shift to and it has much better support.
Types
So within python there are a couple of basic types of data that we can work with and it’s important to understand the differences. The main types of data you will work with are numbers, strings, and booleans.
a number is very simple, it’s a integer like 12 or a floating point number like
123.5 (floating point number is a computer science term for number with a decimal
point)
Math between the two works as expected
Then there are strings, now a computer only understands numbers it has no concept of text, so some engineers came up with something called ascii which gives every letter a number value. This lets us do things like add strings together and edit characters etc. and then just convert the numbers to letters when printing to a screen.
Dec Hex Dec Hex Dec Hex Dec Hex Dec Hex Dec Hex Dec Hex Dec Hex
0 00 NUL 16 10 DLE 32 20 48 30 0 64 40 @ 80 50 P 96 60 ` 112 70 p
1 01 SOH 17 11 DC1 33 21 ! 49 31 1 65 41 A 81 51 Q 97 61 a 113 71 q
2 02 STX 18 12 DC2 34 22 " 50 32 2 66 42 B 82 52 R 98 62 b 114 72 r
3 03 ETX 19 13 DC3 35 23 # 51 33 3 67 43 C 83 53 S 99 63 c 115 73 s
4 04 EOT 20 14 DC4 36 24 $ 52 34 4 68 44 D 84 54 T 100 64 d 116 74 t
5 05 ENQ 21 15 NAK 37 25 % 53 35 5 69 45 E 85 55 U 101 65 e 117 75 u
6 06 ACK 22 16 SYN 38 26 & 54 36 6 70 46 F 86 56 V 102 66 f 118 76 v
7 07 BEL 23 17 ETB 39 27 ' 55 37 7 71 47 G 87 57 W 103 67 g 119 77 w
8 08 BS 24 18 CAN 40 28 ( 56 38 8 72 48 H 88 58 X 104 68 h 120 78 x
9 09 HT 25 19 EM 41 29 ) 57 39 9 73 49 I 89 59 Y 105 69 i 121 79 y
10 0A LF 26 1A SUB 42 2A * 58 3A : 74 4A J 90 5A Z 106 6A j 122 7A z
11 0B VT 27 1B ESC 43 2B + 59 3B ; 75 4B K 91 5B [ 107 6B k 123 7B {
12 0C FF 28 1C FS 44 2C , 60 3C < 76 4C L 92 5C \ 108 6C l 124 7C |
13 0D CR 29 1D GS 45 2D - 61 3D = 77 4D M 93 5D ] 109 6D m 125 7D }
14 0E SO 30 1E RS 46 2E . 62 3E > 78 4E N 94 5E ^ 110 6E n 126 7E ~
15 0F SI 31 1F US 47 2F / 63 3F ? 79 4F O 95 5F _ 111 6F o 127 7F DEL
the word “HELLO” would be [72, 69, 76, 76, 79]. Of course writing out numbers whenever we want to print text is REAL annoying so we can just use a short hand by specifing our string with a double or single quote.
"HEllo"
'woah'
Now python makes is so we never have to think about how strings are really just numbers in the backend, But it’s important to understand this table because when you do something like
"hello" >
"HELLO"
you get True because when python compares every letter of the first string to
the second string it just compares the ascii value, and 104 > 72
We also have another type, boolean, this consists of True and False, self explanatory, it’s for when you want to represent data that can be in one of two states, true or false.
Now you have to be careful because some operations only make sense when done with the same type
"A" + "B" = "AB"
1 + 1 = 2
1 + "A" = ????
python
This will likely be a source of pain for you if you read in the string "1"
but want to add 1 to it as if it was a number. Luckily python lets you convert between things easily.
The string “12” can be converted to the number 12 by just doing int("12")
and similarily you can do float("12.34") and even str(12)to get “12”
One last type that’s important to know is None this is how python represents no data,
it’s important to have this because there is a difference between having 0 of something
and not knowing what the value is, use None whenever you want to signal that
you don’t know what something is.
Variables
Now it would be useless if all python could so it add some numbers together or print some strings out, no better then a calculator, so python lets us store values in variables. These are basically ways we can talk about peices of data by reference
> a = 1
> b = 2
> a + b
3
It is also good to use comments, these are things you can write into your code that are ignored by the interperter, they are there to help other humans understand your code.
# I am doing 1+1
a = 1+1
Comparisons
There are operations that take two things and return a boolean representing some test.
1 > 2 # false, 1 is not greater then 2
1 == 2 # false 1 is not 2
1 != 2 # true! 1 is not 2
not 1 == 2 # true, 1 == 2 is false, not false is true
Functions
Now we can talk about functions, these are bits of code that we can invoke with some params
that allow us to reuse code.
lets start with a simple example, lets say we wanted to
write a function to add 2 numbers together. In simple mathematical notation we’d say
f(x,y) = x + y
i.e we have a function that takes 2 paramaters, x and y and results in x + y. The way we write this in python is
def f(x,y):
return x + y
Well in fact the f can be any name we want the function to have, such as
def add_numbers(x,y):
return x + y
anyway the def says “we are defining a new function”, the name of the function is “f”,
it takes in 2 things called x and y and it returns x + y
Now it may seem weird that we have that colon and then the new line, why not do
def f(x,y): return x+y
well the issue comes when you have more complex functions, lets write a function that returns 2*x + y
def f(x,y): x = 2*x return x+y
This is confusing, where does 1 line of code end and the next begin? are we doing 2*(x return x+y)?
at what point have we ended our function definition and are now writing a different line of code?
For these reasons we have to be mroe verbose, we put a colon to indicate a code block
is coming so the interperter knows that everything after this colon that is indented
is part of this function and once the indentation ends the function is over.
def f(x,y):
x = 2*x
return x+y
Ok now we have a function, how do we use it? You can simply invoke it as
> f(1,2)
4
our function returned 4!
Now other then working in the shell it’s usefull to write all this code in a file and run that,
how would we get our output then?
We can use the print function, this is just like any function we would write
but it’s already been written by a coder and put into our enviornment
print("Hello World")
It takes in a string and shoots it out onto the screen. We can also give print a number
x = f(1,2)
print(x)
Now what do you think the following bit of code will print?
def set_a_to_5(a,b):
a = 5
a = 1
set_a_to_5(a)
print(a)
We get 1, because when a function starts it copies the value of whatever we pass to it into a new variable which gets killed once the function ends. The code inside the function runs in a different scope
python
Finally sometimes you do want a function that is just 1 line long, something simple,
we call these lambdas (stolen from lambda calculus)
> f = lambda x,y: x + y
> f(1,2)
3
These have their own quirks such as closure which we won't get into right now