Basics

Variables

Places to store data and modify it. They are classes and have methods.

Types of variables

a = 1     # integer
b = 2.5   # float
c = "3"   # string
d = '4'   # string
e = False # boolean

Must start with a letter or the underscore. Can not start with a number.
Can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ ).
They are case sensitive.

True or false

a = 0 ## False
a = 1 ## True
a = "" ## False
a = "a" ## True

Comparison Operators

x == y # Equals
x >= y # Greater-than-or-equal-to
x <= y # Less-than-or-equal-to

Assignment Expressions

PEP

x := y # Becomes
x +=  y # In-place add
x -=  y # In-place minus
x *=  y # In-place multiply
x @=  y # In-place matrix multiply
x /=  y # In-place division
x //= y # In-place int division
x %=  y # In-place mod
x &=  y # In-place bitwise and
x |=  y # In-place bitwise or
x ^=  y # In-place bitwise xor
x <<= y # In-place left shift
x >>= y # In-place right shift
x **= y # In-place power

env_base = os.environ.get("PYTHONUSERBASE", None)
if env_base:
    return env_base
# Instead
if env_base := os.environ.get("PYTHONUSERBASE", None):
    return env_base

Weird things

n = []
n += range(3)

def query(url):
    r = requests.get(url)
    if r.status_code == 200:
        return json.loads(r.text)
---
def query(url):
    if 200 == (r := requests.get(url)).status_code:
        return json.loads(r.text)

Multiple assignment

a = b = c = 7

d, e, f, g = 5, 6.7, "8", True

h,i,j = (3,2,1)

tuple = [1, 0.6, "a"]
l, m, n = tuple

* can be at any place but only once.

a, b, *c, d = [1,2,3,4,5,6,7]
print(a) ## 1
print(b) ## 2
print(c) ## [3, 4, 5, 6]
print(d) ## 7

Data types

variable = "Hello"
print(type(variable))  ## <class 'str'>

Check if a variable is a certain type.

if isinstance(variable, str):
    print(f"'{variable}' is a string")

Variable types are: - basestring string and unicode - str string - int integer - bool boolean - float float

Other data types are: - https://www.mssqltips.com/sqlservertip/7126/learn-python-data-types/ - https://www.mssqltips.com/sqlservertip/7127/python-list-tuple-range-dictionary-and-set-data-type-examples/

Lists `[]`

Ordered
Indexed
Mutable
Non-homogeneous
Allows duplicates

Dynamically sized array.

["a",1,3.5]

my_list = list()
empty_list = []

Tuples

Ordered
Immutable
Non-homogeneous
Allows duplicates

Collection of various objects separated by commas.

("a",1,3.5)

my_tuple = tuple()
empty_tuple = ()

Sets

Mutable
Unordered
Un-indexed
Non-homogeneous
Does not allow duplicates (unique)
Iterable
Nice for searching
Faster than the rest

Unordered collection of data types.

{"a",1,3.5}

my_set = set()

set.union(my_set2)
set.intersection(my_set2)

Dictionary

Docs

Mutable
Unordered
Unique key (not duplicated)
Non-homogeneous

Unordered collection of various data types. Used for maps.

my_dic = dic()
empty_dic = {}

my_dic = {"a":10,1:"b",3.5:"z"}
print(my_dic["a"]) # 10
print(my_dic[3.5]) # "z"

if "b" in i:
    print(i["b"])

Dictionary methods

i = {"a":1,"b":2,"c":3,"d":4,"e":5,"f":6,"g":7}
for j in i:
    print(j) ## Print the keys

for k,v in i.items():
    print(f"{k}:{v}")

for j in i.values():
    print(j)

for j in i.keys():
    print(j)

print(i)
i.pop("d")
print(i)

print(i["a"])
print(i.get("a"))

Mutable or immutable

Source List of immutable types:

int, float, decimal, complex, bool, string, tuple, range, frozenset, bytes

List of mutable types:

list, dict, set, bytearray, user-defined classes

Casting

Explicitly specify the type of the variable.

str(7)    ## "7"
float(7)  ## 7.0
int(7.7)  ## 7
bool(0)   ## False

Convert from one type to other (int to str):

a = 3 ## int
print("a is " + str(a)) ## Concadenate two strings

Strings

a = ""  ## False
a = "a" ## True

String slicing

indexing[] [start:stop:step]
slice()
The indexes as exclusive, [0:8] will go from 0 to 7.
Every character has a positive and a negative index.

Indexing. You need to do something with the resulting string.

print("string"[:3]) ## "str"
s = "The quick brown fox jumps over the lazy dog"
s[0]    ## First letter
s[-1]   ## Last letter
s[0:3]  ## First 3 letters. From 0 to 2
s[:3]   ## First 3 letters (short). From 0 to 2
s[-3:]  ## Last 3 letters
s[::2]  ## Print even characters
s[::-1] ## Reverse the string

Slicing

s = "The quick brown fox jumps over the lazy dog"
print(s[slice(1)])   ## First letter
print(s[slice(0,3)]) ## First 3 letters
print(s[slice(4,-4)]) ## "quick brown fox jumps over the lazy"
print(s[slice(-1)])  ## Everything except the last letter
print(s[slice(0,len(s),2)]) ## Even characters

p = slice(0,3)
print(s[p])

Example

s = "The quick brown fox jumps over the lazy dog"
p = s.find(" ") ## Find the first space
print(s[:p])    ## Print the first word

Conditional statements (ternary operators)

> Greater than
>= Greater than or equal
== Equals
!= Not equals
< Less than
<= Less than or equal

They resolve to True or False.

Example

age = 45
if age > 34:
    print("You are older than 34")

age >= 34 ## Prints "True"

Logical operators

Used to combine conditional statements.

and both statements have to be True
or only one statement has to be True
not invert

Example. max_temp has to be greater than 22 and rain has to be False

if not (max_temp > 22 and not rain):
    print("It is going to be a cold and rainy")

if...elif...else

One line if

a = 2
b = 20
if a < b: print("a is smaller than b")

One line if...else

b = 7
a = "positive" if b >= 0 else "negative"

One line if...elif...else

b = 7
a = "neg" if b < 0 else "pos" if b > 0 else "zero"

if b < 0:
   a = "neg"
elif b > 0:
   a = "pos"
else:
   a = "zero"

Nested if. if statement inside an if statement.

if x > 10:
    if x == 45:
        print("It is 45!")
    else:
        print("Is bigger than 10")

Loops

For

radio_names = []
for s in radio_stations:
    radio_names.append(s[0])

## Equal to
radio_names = [s[0] for s in radio_stations]

Classes

Methods

Function which is a member of a class. A function inside a class

Find all the methods for str()

print(dir(str()))

https://www.askpython.com/python/examples/find-all-methods-of-class

Built-in Functions

Source

type()
dir()
id()
getattr()
hasattr()
globals()
locals()
callable()

Functions

Multi-Return

def test(par1,par2,par3):
    return par1,par2,par3

name,surname,birth = test('john','smith',1976)

print(name)
print(surname)
print(birth)

Default value for arguments in a function

def test(par1,par2=None):
    ret1 = par1
    ret2 = par2
    return ret1,ret2

name,surname = test('john','smith')

print(name)
print(surname)

name,surname = test('john')

print(name)
print(surname)

`n` arguments for a function.

def test( par1='', **atr ):
    return par1,atr

name,atrs = test('john',height=180,country='Italy',city='Bonachello')

print(name)
print(atrs)

for key, val in atrs.items():
    print('[%s] => [%s]' % (str(key), str(val)))

print(atrs['country'])

name,atrs = test(height=180,country='Italy',city='Bonachello')

try:
    if atrs['city']:
        print(f"The city is {atrs['city']}")
except KeyError:
    print("There is no city")

try:
    city = atrs['city']
except KeyError:
    print("There is no city")
else:
    print(f"The city is {city}")

try:
    city = atrs['city']
except KeyError:
    city = False

Weird things

Dynamically create variables named as the values in a list

Source

Variable names can not start with an int, that is why the f"id_{i}".

class Test:
    def __init__(self,id,type,price,name="null"):
        self.id = id
        self.type = type
        self.price = price
        self.name = name

ids = [1234,2345,3456,4567]

## Assign
for i in ids:
    globals()[f"id_{i}"] = Test(i,"type",1.2,"name")

## Print all
for i in ids:
    print(globals()[f"id_{i}"].id)
    print(globals()[f"id_{i}"].type)
    print(globals()[f"id_{i}"].price)
    print(globals()[f"id_{i}"].name)

## Print individually
print(id_1234)
print(id_2345)
print(id_3456)
print(id_4567)

Modules

Web server

Python 3

python -m http.server
python -m http.server --cgi

Python 2

python -m SimpleHTTPServer

Conventions and Terminology

PEP: Python Enhancement Proposals

Run old versions of python.

I found to be easiest with Conda.

conda create -n your_env_name python=3.9
conda activate your_env_name
conda install package_name
pip install package_name

SSL Certificates

For some reason it does not use the ones installed in the system.

conda config --set ssl_verify <pathToYourFile>.crt

pip install flask cassandra-driver --trusted-host pypi.org --trusted-host pypi.python.org --trusted-host files.pythonhosted.org
pip config --global set global.trusted-host "pypi.org pypi.python.org files.pythonhosted.org"

pip install flask --cert=/usr/local/share/ca-certificates/proxy.crt

Sources

https://www.golinuxcloud.com/python-if-else-one-line/

Basics

Variables

Types of variables

True or false

Comparison Operators

Assignment Expressions

Multiple assignment

Data types

Lists []

Tuples

Sets

Dictionary

Mutable or immutable

Casting

Strings

String slicing

Conditional statements (ternary operators)

Logical operators

if...elif...else

Loops

For

Classes

Methods

Built-in Functions

Functions

Multi-Return

Default value for arguments in a function

n arguments for a function.

Weird things

Dynamically create variables named as the values in a list

Modules

Web server

Conventions and Terminology

Run old versions of python.

SSL Certificates

Sources

Lists `[]`

`n` arguments for a function.