francoiducat/pythonconfinement

python Confinement Course

General Commands

• ctrl + l : clear python shell
• exit() : exit python shell
• python3 my_python_script.py : run a python script (for osx users)

Types

Integers are for representing whole numbers:

rank = 10
eggs = 12
people = 3

Floats represent continuous values:

temperature = 10.2
rainfall = 5.98
elevation = 1031.88

Strings represent any text:

message = "Welcome to our online shop!"
name = "John"
serial = "R001991981SW"

Lists represent arrays of values that may change during the course of the program

members = ["Sim Soony", "Marry Roundknee", "Jack Corridor"]
pixel_values = [252, 251, 251, 253, 250, 248, 247]

Dictionaries represent pairs of keys and values

phone_numbers = {"John Smith": "[phone]", "Marry Simpons": "[phone]"}
volcano_elevations = {"Glacier Peak": 3213.9, "Rainer": 4392.1}

Keys of a dictionary can be extracted with

phone_numbers.keys()

Values of a dictionary can be extracted with

phone_numbers.values()

Tuples represent arrays of values that are not to be changed during the course of the program

vowels = ('a', 'e', 'i', 'o', 'u')
one_digits = (0, 1, 2, 3, 4, 5, 6, 7, 8, 9)

To find out what attributes a type has

dir(str)
dir(list)
dir(dict)

Find out what Python builtin functions there are

dir(__builtins__)

Documentation for a Python command can be found with

help(str)
help(str.replace)
help(dict.values)

Positive/Negative Indexes, Slicing

Lists, strings, and tuples have a positive index system:

["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
   0      1      2      3      4      5      6

And a negative index system:

["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
  -7     -6     -5     -4     -3     -2     -1

In a list, the 2nd, 3rd, and 4th items can be accessed with

days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
days[1:4]

Output: ['Tue', 'Wed', 'Thu']

First three items of a list

days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
days[:3]

Output:['Mon', 'Tue', 'Wed']

Last three items of a list

days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
days[-3:]

Output: ['Fri', 'Sat', 'Sun']

Everything but the last

days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
days[:-1] 

Output: ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat']

Everything but the last two

days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
days[:-2]

Output: ['Mon', 'Tue', 'Wed', 'Thu', 'Fri']

A single in a dictionary can be accessed using its key

phone_numbers = {"John Smith":"[phone]","Marry Simpons":"[phone]"}
phone_numbers["Marry Simpsons"]

Output: '[phone]'

Conditionals

Using "and" and "or" in a Conditional

Check for one single condition

x = 1
 
if x == 1:
    print("Yes")
else:
    print("No")

Check if two conditions are met at the same time using an and operator

x = 1
y = 1
 
if x == 1 and y==1:
    print("Yes")
else:
    print("No")

That will return Yes since x == 1 and y ==1 are both True.

You can also check if one of two conditions are met using an or operator

x = 1
y = 1
 
if x == 1 or y==2:
    print("Yes")
else:
    print("No")

That will return Yes since at least one of the conditions is True. In this case x == 1 is True.

Functions and Conditionals

Define a function

def cube_volume(a):
    return a * a * a

Write a conditional block

message = "hello there"
 
if "hello" in message:
    print("hi")
else:
    print("I don't understand")

Write a conditional block of multiple conditions:

message = "hello there"
 
if "hello" in message:
    print("hi")
elif "hi" in message:
    print("hi")
elif "hey" in message:
    print("hi")
else:
    print("I don't understand")

Use the and operator to check if both conditions are True at the same time:

x = 1
y = 1
 
if x == 1 and y==1:
    print("Yes")
else:
    print("No")

Output is Yes since both x and y are 1.

Use the or operator to check if at least one condition is True:

x = 1
y = 2
 
if x == 1 or y==2:
    print("Yes")
else:
    print("No")

Output is Yes since x is 1.

Check if a value is of a certain type with:

isinstance("abc", str)
isinstance([1, 2, 3], list)

or

type("abc") == str
type([1, 2, 3]) == lst

Processing User Input

A Python program can get user input via the input function

The input function halts the execution of the program and gets text input from the user

name = input("Enter your name: ")

The input function converts any input to a string, but you can convert it back to int or float

experience_months = input("Enter your experience in months: ")
experience_years = int(experience_months) / 12

You can format strings with (works both on Python 2 and 3)

name = "Sim"
experience_years = 1.5
print("Hi %s, you have %s years of experience." % (name, experience_years))

Output: Hi Sim, you have 1.5 years of experience.

You can also format strings with (Python 3 only)

name = "Sim"
experience_years = 1.5
print("Hi {}, you have {} years of experience".format(name, experience_years))

Output: Hi Sim, you have 1.5 years of experience.

For Loop Over a Function

Call any function multiple times, even your own functions.

Let's suppose we defined this function:

def celsius_to_kelvin(cels):
    return cels + 273.15

That is a function that gets a number as input, adds 273.15 to it and returns the result.

A for loop allows us to execute that function over a list of numbers

monday_temperatures = [9.1, 8.8, -270.15]
 
for temperature in monday_temperatures:
    print(celsius_to_kelvin(temperature))

The output of that would be:

• 282.25
• 281.95
• 3.0

So, in the first iteration celsius_to_kelvin(9.1) was executed, in the second celsius_to_kelvin(8.8) and in the third celsius_to_kelvin(-270.15).

Dictionary Loop and String Formatting

Combine a dictionary for loop with string formatting to create text containing information from the dictionary:

phone_numbers = {"John Smith": "[phone]", "Marry Simpons": "[phone]"}
 
for pair in phone_numbers.items():
    print("{} has as phone number {}".format(pair[0], pair[1]))

Another (better) way to do it::

phone_numbers = {"John Smith": "[phone]", "Marry Simpons": "[phone]"}
 
for key, value in phone_numbers.items():
    print("{} has as phone number {}".format(key, value))

In both cases the output is:

• John Smith has as phone number [phone]
• Marry Simpons has as phone number [phone]

Loops

Loops are useful for executing a command over a large number of items.

for letter in 'abc':
    print(letter.upper())

Output:

• A
• B
• C

The name after for (e.g. letter) is just a variable name

Loop over dictionary keys

phone_numbers = {"John Smith":"[phone]","Marry Simpons":"[phone]"}
for value in phone_numbers.keys():
    print(value)

Output:

• John Smith
• Marry Simpsons

Loop over dictionary values

phone_numbers = {"John Smith":"[phone]","Marry Simpons":"[phone]"}
for value in phone_numbers.values():
    print(value)

Output:

• [phone]
• [phone]

Loop over dictionary items

phone_numbers = {"John Smith":"[phone]","Marry Simpons":"[phone]"}
for key, value in phone_numbers.items():
    print(key, value)

Output:

• ('John Smith', '[phone]')
• ('Marry Simpons', '[phone]')

While loops will run as long as a condition is true:

while datetime.datetime.now() < datetime.datetime(2090, 8, 20, 19, 30, 20):
    print("It's not yet 19:30:20 of 2090.8.20")

The loop above will print out the string inside print() over and over again until the 20th of August, 2090.

List Comprehensions

A list comprehension is an expression that creates a list by iterating over another container.

A basic list comprehension

[i*2 for i in [1, 5, 10]]

Output: [2, 10, 20]

List comprehension with if condition

[i*2 for i in [1, -2, 10] if i>0]

Output: [2, 20]

List comprehension with an if and else condition

[i*2 if i>0 else 0 for i in [1, -2, 10]]

Output: [2, 0, 20]

More on Functions

Functions can have more than one parameter

def volume(a, b, c):
    return a * b * c

Functions can have default parameters (e.g. coefficient)

def converter(feet, coefficient = 3.2808):
    meters = feet / coefficient
    return meters
 
print(converter(10))

Output: 3.0480370641306997

Arguments can be passed as non-keyword (positional) arguments (e.g. a) or keyword arguments (e.g. b=2 and c=10)

def volume(a, b, c):
    return a * b * c 
print(volume(1, b=2, c=10))

An *args parameter allows the function to be called with an arbitrary number of non-keyword arguments

def find_max(*args):
    return max(args)
print(find_max(3, 99, 1001, 2, 8))

Output: 1001

An **kwargs parameter allows the function to be called with an arbitrary number of keyword arguments

def find_winner(**kwargs):
    return max(kwargs, key = kwargs.get)
 
print(find_winner(Andy = 17, Marry = 19, Sim = 45, Kae = 34))

Output: Sim

Summary of function elements:

File Processing

Read an existing file with Python

with open("file.txt") as file:
    content = file.read()

Create a new file with Python and write some text on it

with open("file.txt", "w") as file:
    content = file.write("Sample text")

Append text to an existing file without overwriting it

with open("file.txt", "a") as file:
    content = file.write("More sample text")

Both append and read a file with

with open("file.txt", "a+") as file:
    content = file.write("Even more sample text")
    file.seek(0)
    content = file.read()

Imported Modules

Builtin objects are all objects that are written inside the Python interpreter in C language.

Builtin modules contain builtins objects.

Some builtin objects are not immediately available in the global namespace. They are parts of a builtin module. To use those objects the module needs to be imported first. E.g.:

import time
time.sleep(5)

A list of all builtin modules can be printed out with:

import sys
sys.builtin_module_names

Standard libraries is a jargon that includes both builtin modules written in C and also modules written in Python.

Standard libraries written in Python reside in the Python installation directory as .py files. You can find their directory path with sys.prefix.

Packages are a collection of .py modules.

Third-party libraries are packages or modules written by third-party persons (not the Python core development team).

Third-party libraries can be installed from the terminal/command line:

Windows:

pip install pandas

Mac and Linux:

pip3 install pandas

The any() function

if not any(website in line for website in website_list) part in the previous video, here is another example:

>>> lines  = ["trees are good", "pool is fresh", "face is round"]
>>> website_list = ["face", "clock", "trend"]
>>> for line in lines:
...     any(website in line for website in website_list)
... 
False
False
True

We start iterating over the items of website_list using a for loop. In the first iteration we would have:

any(website in "trees are good" for website in website_list)

Inside the parenthesis of any() there's another loop that iterates over website_list:

("face" in "trees are good")
("clock" in "trees are good")
("trend" in "trees are good")

If any of the above is True you get the expression evaluated to True. In this case none of them is True, so you get False.

If you want to return True (if all of them are True), use all() instead of any().

So, the part any(website in line for website in website_list) will either be equal to True or False.

Useful commands

Check wich modules are installed

pip3 freeze

Check python version of the terminal you're using

For some reasons, an imported module in your favorite terminal may be not found in the terminal of your favorite IDE. It's then interested to check the python version of IDE's terminal.

Francois at Mycomputer in ~/WorkspacePython/app4/webapp (master●●)
$ python3
Python 3.7.6 (default, Dec 30 2019, 19:38:26) 
[Clang 11.0.0 (clang-1100.0.33.16)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.version
'3.7.6 (default, Dec 30 2019, 19:38:26) \n[Clang 11.0.0 (clang-1100.0.33.16)]'
>>> sys.executable
'/usr/local/opt/python/bin/python3.7'
>>> 

Both python version should be the same (3.7 in this specific case)