Python

Basics

Variables

Declaring Variables:

list=[1,2,3]
string="Hello"
int=23

Basic argument specifiers:

%s                      - String (or any object with a string representation, like numbers)
%d                      - Integers
%f                      - Floating point numbers
%.<number of digits>f   - Floating point numbers with a fixed amount of digits to the right of the dot.
%x/%X                   - Integers in hex representation (lowercase/uppercase)

Indentation is Mandatory in python:

num=12
if num>5:
 print("Number is greater than 5")
 if num<20:
   print("Number is lesser than 20")
print("Program Ended")

Operator Precedence from highest to lowest order:

Statements

IF & ELIF statements:

num=9
if num==5:
 print("Number is 5")
elif num ==11:
 print("Number is 11")
elif num==7:
 print("Number is 7")
else:
 print("Number isn't 5,11,7")

AND returns true only if both are true

>>> 1 == 1 and 2==2
True
>>> 1 == 1 and 2==3
False
>>> 1 != 1 and 2==2
False
>>> 1 == 1 and 2==2
True

OR returns true if either of both arguments are true; false if both are false.

>>> 1 != 1 or 2==2
True
>>> 1 == 1 or 2==2
True
>>> 1 == 1 or 2==3
True

Lists

words=["Hello","World","!"]
print(words[0])
print(words[1])
print(words[2])

Mixed List:

number=3
things=['string',0,[1,2,number],4,56]
print(things[1])
print(things[2])
print(things[2][2])

List Operators:

nums = [1,2,3]
print(nums+[4,5,6])
print(nums*3)

in operator:

>>> words=['spam',"egg"]
>>> print("spam" in words)
True
>>> print("tomato" in words)
False
>>> print (not "tomato" in words)
True
>>> print ("tomato" not in words)
True

Append to the list:

nums=[1,2,3]
nums.append(4)
print(nums)

Len function:

1. Unlike append, len is a normal function rather than a method.
2. Therefore it is written before the list it is being called on, without a dot.

nums=[1,3,5,2,4]
print(len(nums))

Insert Method is similar to append, but allows to add at any position in the list

words=["Python","fun"]
index=1
words.insert(index,"is")
print(words)

Tuple

• Tuples are immutable which means you cannot update or change the values of tuple elements.
• You are able to take portions of existing tuples to create new tuples.
• Tuples cannot be changed unlike Lists.
• They use parentheses.

• Declaring tuples:

tup1 = ()
tup2 = (50,)
tup3 = ('physics', 'chemistry', 1997, 2000)
tup4 = (1, 2, 3, 4, 5 )
tup5 = "a", "b", "c", "d"

• Printing tuples:

tup1[0]
print "tup2[1:5]: ", tup2[1:5]

• Adding tuples:

tup1 = (12, 34.56)
tup2 = ('abc', 'xyz')

tup3 = tup1 + tup2
print (tup3)
(12, 34.56, 'abc', 'xyz')

• Following action is not valid for tuples

  tup1[0] = 100 

Dictionary

• How a dictionary looks like:

{'abc': '43f9', 'sample': 'fc9e'}

• Declaring a Dictionary:

x = {}

• Adding Entry:

x.update({"abc": "112"})

• Another example for adding Entry:

for file in fullpath(dir):
       filesize = os.path.getsize(file)
       print(file, filesize)
       x.update({file: filesize})

• Dictionary Comprehension:

sq = {x: x*x for x in range(10)}
print(sq)

for i in sq:
   print(sq[i])

• Printing Dictionary:

>>> x.items()
dict_items([('a', 12), ('b', 14), ('c', 16), ('d', 18)])

• Printing items from dictionary:

for k,v in x.items():
   print(k,v)

• Printing specific entries:

x['a']
x[4]

• Length of Dictionary:

len(x)

Arrays

• Declaring Arrays:

x = [1,2,4,6,8,9]
x = [i for i in range(10)]
x = [i*i for i in range(10)]

• Appending values:

x = []
for i in range(100):
  x.append(i)

• Printing specific values:

print(x[20])

Loops

While Loop runs more than once, till condition is true, once condition is false, next section of code is executed:

i=1
while i<=500:
 print(i)
 i=i+1

print("Finished !")

Infinity while loop: condition always remain True:

while 1==1:
 print('In a loop..!!')

Break the infinity Loop:

i=0
while 1==1:
 print(i)
 i=i+1
 if i>=25:
  print('Breaking!')
  break

print("Finished!")

Continue this jumps back to top of the loop, rather than stopping it

i=0
while True:
 i=i+1
 if i==10:
  print('Skip 10')
  continue
 if i==25:
  print("Breaking")
  break
 print(i)
print("Finished!")

While Loop

words = ["hello","world","spam","egg"]
counter = 0
max_index = len(words)-1

while counter <= max_index:
 word = words[counter]
 print(word + "!")
 counter = counter + 1

Same code can be created using FOR Loop but fewer lines

words = ["hello","world","spam","egg"]
for word in words:
 print(word + "!")

Loop can be combined with Range objects(No need to call List on range objects in For Loop as it is not indexed):

for i in range(5):
 print("hello")

Functions

def myfunc():
 print("spam")
 print("spam")
 print("spam")

myfunc()

Arguments

def printe(word):
 print(word + "!")

printe("spam")
printe("world")
printe("egg")

Arguments: Functions can have more that 1 argument:

def print_twice(x,y):
 print(x+y)
 print(x+y)

print_twice(5,8)

Function arguments can be used as variables inside function, not outside:

def incr(x):
 x += 1
 print(x)

incr(7)
print(x)

Return: int & str functions return a value which can be used later: Return cannot be used outside of a function.

def max(x,y):
 if x>=y:
  return x
 else:
  return y

print(max(4,7))
z = max(8,5)
print(z)

def short(x,y):
 if len(x) <= len(y):
   return x
 else:
   return y

After using return in a function, it stops executing immediately:

def add(x,y):
 total = x + y
 return total
 print("This wont be printed")

print(add(4,5))

Lambda Function

>>> mul = lambda x: x**2 + 2*x-5
>>> mul(5)
30

>>> add = lambda y: a + b
>>> a=6
>>> b=5
>>> add (10)
11

>>> add2 = lambda a,b: a + b
>>> add2(10,3)
13

>>> rem = lambda a,b: a%b
>>> add (10,3)
1

Comments

#This is a comment.
print("Test") #This is also a comment.
#print("Test 2")

Docstrings:

def excl(word):
 """
 Print a word
 with exclamation!
 """
 print(word + "!")

shout("spam")

Functions as an Object: they can be assigned & reassigned to variables.

def multiply(x,y):
 return x*y

a = 4
b = 7
operation = multiply
print(operation(a,b))

def shout(word):
 return word + "!"
speak = shout
output = speak("shout")
print(output)

Function can be used as arguments of other functions:

def add(x,y):
 return x + y

def twice(func(x,y)):
 return func(func(x,y), func(x,y))

a=6
b=12

print(twice(add,a,b))

def square(x):
 return x*x

def test(func,x):
 print(func(x))

test(square,42)

Modules

Modules are piece of code written by others to fulfil common tasks.

Print 5 random numbers in range 1 to 8:

import random
for i in range(5):
 value = random.randint(1,8)
 print(value)

import math
num = 10
print(math.sqrt(num))

Import certain functions only from module:

from math import pi
print(pi)

from math import pi,sqrt
from math import *            # not recommended, can confuse the variables in code with variables external module.

from math import sqrt as square_root
print(square_root(100))

3 types of Modules:

Standard Library -> string, re, datetime, math, random, os, multiprocessing, subprocess, socket, email, json, doctest, unitest, pdb, argparse, sys
Installed from external sources
Manually written

Python Package Index

PyPI is used to install 3rd party modules.

sudo apt-get install python-pip

pip install library_name
pip install paramiko

List modules:

pip list
pip freeze
help('modules')

Exceptions

ImportError: an import fails
IndexError: list is indexed with an out of range  number
NameError: an unknown variable is used
SyntaxError: the code can't be parsed properly
TypeError: a function is called on a value of inappropriate type
ValueError: a function is called on value of correct type but with inappropriate value
OSError:

Third party libraries has their own defined exceptions.

ZeroDivisionError:

 x = 7
 y = 0
 print(x/y)

Exception Handling

try:
 x = 7
 y = 0
 print(x/y)
 print("Calc Done")
except ZeroDivisionError:
 print("An Error Occured")
 print("due to zero division")

Handling multiple exceptionin a single except statement:

try:
 x = 10
 print(x + "Hello")
 print(x / 2)
except ZeroDivisionError:
 print("Division by Zero")
except(ValueError, TypeError):
 print("Error occured")

Catch all error(not recommended, they can catch unexpected errors and hide programming mistakes): Exception handling is quite useful when dealing with user input.

try:
 word = "spam"
 print(word / 0)
except:
 print("An error occured")

Finally: this code will always run, even if uncaught exception occurs.

try: 
 print("Hello")
 print(1/0)
except ZeroDivisionError:
 print("Div by Zero")
finally:
 print("This will always run")

try:
 print(1)
 print(10/0)
except ZeroDivisionError:
 print(unknown_var)
finally:
 print("This is executed last")

Raising Exceptions:

print(1)
raise ValueError
print(2)

name = "123"
raise NameError("Invalid name!")

x = input(":")
if float(x) < 0:
 raise ValueError("Negative!")

Raise statement can be used to raise whatever exception occurs:

try:
 x = 5/0
except:
 print("An error occured")
 raise

Assertions

Used for sanity checking, programmers use this at start of fuction to check for valid input and after function call to check for valid output.
Expression is tested, if result is false, an exception is raised.

print(1)
assert 2+2 == 4
print(2)
assert 1 + 1 == 3
print(3)

Assertons can take a second argument passed to AssertionError if assertion fails:
Assertions can be caught & handled like other exceptions using Try-Except statement.
If not handled, it can terminate the program.

temp = -10
assert (temp >= 0),"Colder than absolute Zero!"

Exercise

Function that returns sum of all numbers from 0 till that number:

def sum(x):
 res=0
 for i in range(x):
 res+=1
 return res

What is highest nymber output by this code?

def nums(x):
 for i in range(x):
  print(i)
  return
nums(10)

What is the output of this code?

def func(x):
 res = 0
 for i in range(x):
  res += i
 return res

print(func(4))

What is the highest number printed by this code:

print(0)
assert "h" != "w"
print(1)
assert False
print(2)
assert True
print(3)

Running Unix Commands

os.system(deprecated)

import os
os.system("date")

 import os
 f = os.popen('date')
 now = f.read()
 print "Today is ", now

Subprocess

Basic usage:

import subprocess
subprocess.call("command1")
subprocess.call(["command1", "arg1", "arg2"])

Execute the date command:

import subprocess
subprocess.call("date")

Execute ls command with arguments:

import subprocess
subprocess.call(["ls", "-l", "/etc/resolv.conf"])

Execute date command:

import subprocess
p = subprocess.Popen("date", stdout=subprocess.PIPE, shell=True)
(output, err) = p.communicate()
print "Today is", output

import subprocess
p = subprocess.Popen(["ls", "-l", "/etc/resolv.conf"], stdout=subprocess.PIPE)
output, err = p.communicate()
print "*** Running ls -l command ***\n", output

import subprocess
p = subprocess.Popen(["ping", "-c", "10", "4.2.2.2"], stdout=subprocess.PIPE)
output, err = p.communicate()
print  output

The only problem with above code is that output, err = p.communicate() will block next statement till ping is completed i.e. you will not get real time output from the ping command.
So you can use the following code to get real time output:

import subprocess
cmdping = "ping -c4 4.2.2.2"
p = subprocess.Popen(cmdping, shell=True, stderr=subprocess.PIPE)
while True:
    out = p.stderr.read(1)
    if out == '' and p.poll() != None:
        break
    if out != '':
        sys.stdout.write(out)
        sys.stdout.flush()

Using SSH

import paramiko
ssh = paramiko.SSHClient()
ssh.set_missing_host_key_policy(paramiko.AutoAddPolicy())
ssh.connect( '192.168.1.23', username = 'root', password = 'libreelec' )
stdin, stdout, stderr = ssh.exec_command( 'ls -al' )

for line in stdout:
    print('... ' + line.strip('\n'))
ssh.close()

Using Databases

Installing Modules for MySQL:

sudo apt-get install python-pip python-dev libmysqlclient-dev
pip install MySQL-python

Connecting to a DB on localhost(need to create a db 'testdb' beforehand):
Source: tutorialspoint.com

#!/usr/bin/python
import MySQLdb
db = MySQLdb.connect("localhost","guest","guest123","testdb")

cursor = db.cursor()
cursor.execute("SELECT VERSION()")
data = cursor.fetchone()
print "Database Version : %s " % data
db.close()

Creating Database:

#!/usr/bin/python
import MySQLdb
db = MySQLdb.connect("localhost","guest","guest123","testdb")

cursor = db.cursor()
sql="""CREATE table book_tbl (
       book_id INT NOT NULL AUTO_INCREMENT,
       book_title VARCHAR(100) NOT NULL,
       book_author VARCHAR(100) NOT NULL,
       submission_date DATE,
       PRIMARY KEY( book_id ));"""

cursor.execute(sql)
db.close()

Writing to Database with User Input:

#!/usr/bin/python
import MySQLdb
a=int(input('Please enter the Book ID:\n'))
b=str(raw_input('Please enter the Book Title:\n'))
c=str(raw_input('Please enter the Book Author:\n'))
d=int(input('Please enter the Book Submission Date:\n'))

db = MySQLdb.connect("localhost","guest","guest123","testdb" )

cursor = db.cursor()

sql = "INSERT INTO book_tbl(book_id, \
       book_title, book_author, submission_date) \
       VALUES ('%d', '%s', '%s', '%d' )" % \
       (a, b, c, d )
try:
   cursor.execute(sql)
   db.commit()
   print("Database Updated.")
except:
   db.rollback()
   print("Error when saving to DB.\n")
db.close()

Read from Database:

#!/usr/bin/python
import MySQLdb
db = MySQLdb.connect("localhost","guest","guest123","testdb" )

cursor = db.cursor()

sql = "SELECT * FROM book_tbl WHERE book_id > '%d'" % (10)

try:
  cursor.execute(sql)
  results = cursor.fetchall()
  for row in results:
      ID = row[0]
      Title = row[1]
      Author = row[2]
      Date = row[3]

      print "ID=%d,Title=%s,Author=%s,Date=%s" % (ID, Title, Author, Date )
except:
  print "Error: unable to fetch data"
db.close()

Pillow

• Install in Python3

sudo apt-get install python3-pip
sudo apt-get install python3-dev python3-setuptools
sudo apt-get install libjpeg8-dev zlib1g-dev libwebp-dev tcl8.5-dev tk8.5-dev
sudo pip3 install Pillow

from PIL import Image

im = Image.open("sample.webp").convert("RGB")
im.save("test.jpg","jpeg")

OpenCV

Windows

• Install NumPy

sudo pip3 install numpy

• Download the relevant WHL file from:

https://www.lfd.uci.edu/~gohlke/pythonlibs/#opencv

• Install OpenCV WHL file:

pip install opencv_python‑3.3.1‑cp36‑cp36m‑win32.whl

• Verify installation:

import cv2
print(cv2.__version__)

Linux

sudo pip3 install opencv-contrib-python

Non Recursive files only list

Using Functions(file name only)

dir = sys.argv[1]

def files(path):
    for file in os.listdir(path):
        if os.path.isfile(os.path.join(path, file)):
            yield file

for file in files(dir):
    print(file)

Using Functions(full paths)

dir = sys.argv[1]

def fullpath(path):
    for file in os.listdir(path):
        if os.path.isfile(os.path.join(path, file)):
            yield (os.path.join(path, file))

for file in fullpath(dir):
    print(file)

Without using Functions

dir = sys.argv[1]

files = [f for f in os.listdir(dir) if os.path.isfile(os.path.join(dir, f))]

for file in files:
    print(file)

GUI

Tkinter

Installing:

sudo apt install python-tk
sudo apt install python3-tk

Importing:

try:
    from Tkinter import *     # for Python2
except ImportError:
    from tkinter import *     # for Python3

Importing as per PEP8:

import tkinter as tk
root = tk.Tk()
tk.Button(...) 

Or bit more typing:

import tkinter
root = tkinter.Tk()
tkinter.Button(...)

Hello World

from tkinter import Tk, Label

root = Tk()

header = Label(root, text="Renamer App v1.0")
header.pack()

root.mainloop()

Buttons

from tkinter import Tk, Button

root = Tk()

def backup():
    print("Backup Files")

a = Button(root, text="Backup Files", command=backup)
a.pack()

root.mainloop()

Label

from Tkinter import *
master = Tk()

w = Label(master, text="Hello, world!")
w.pack()

mainloop()

Listbox

Defining Listbox:

listbox = Listbox(root, height=30, width=100)
listbox.xview_scroll(3, "pages")
listbox.yview_scroll(3, "pages")
listbox.grid(row=7, columnspan=3)

Inserting output:

listbox.insert(END, "1st Entry")

Deleting Output:

listbox.delete(0, END)

Auto-scrolling Scrollbar:

from tkinter import *

root = Tk()

scrollbar = Scrollbar(root, orient=VERTICAL)
listbox = Listbox(root, height=10, width=70, yscrollcommand=scrollbar.set)
listbox.xview_scroll(3, "pages")
listbox.yview_scroll(3, "pages")
scrollbar.config(command=listbox.yview)
listbox.grid(row=0, column=0, rowspan=10, columnspan=10)
scrollbar.grid(row=0, column=11, rowspan=10, columnspan=1, sticky=W, ipady = 60)

for i in range(40):
    listbox.insert(END, "This is Test Line no: "+str(i))

root.mainloop()

Input

root = Tk()

en = Entry(root, width=60)
en.pack()
en.focus_set()

mainloop()

Output in CLI

from tkinter import *
master = Tk()

def callback():
    print ("My Message")

Button(master, text="get", width=10, command=callback).pack()

mainloop()

Output in GUI

from tkinter import *
master = Tk()

def callback():
    Label(master, text="Callback Function").pack()

Button(master, text="get", width=10, command=callback).pack()
mainloop()

Pack Manager

from Tkinter import *
root = Tk()

w = Label(root, text="Red", bg="red", fg="white")
w.pack()

Label(root, text="Green", bg="green", fg="black").pack()

mainloop()

Grid Manager

from tkinter import *

root = Tk()

Button(root, text="files", width=20, command="ls").grid(row=0, column=0)
Button(root, text="Clear", width=20, command="clear").grid(row=0, column=1)
Button(root, text="Backup", width=20, command="backup").grid(row=1, column=0)
Button(root, text="Miss", width=20, command="miss").grid(row=1, column=1)

root.mainloop()

Filedialog

import tkinter
from tkinter import filedialog
import os

root = tkinter.Tk()
root.withdraw() #use to hide tkinter window

currdir = os.getcwd()
dir = filedialog.askdirectory(parent=root, initialdir=currdir, title='Please select a directory')
if len(dir) > 0:
    print ("You chose %s" % dir)

Toplevel

Method to create Child Windows:

from tkinter import Button, Toplevel, Tk

root = Tk()

def press():
    win1 = Toplevel()
    Button(win1, text = "Event", command=event).grid()
    Button(win1, text = "Q", command=win1.destroy).grid()

def event():
    print("Event Printed")

Button(root, text = "A", command=press).grid()
root.geometry('200x200')
root.mainloop()

Key Bindings

from tkinter import *
 def hello(event):
    print("A Clicked, Button-l") 
def quit(event):                           
    print("Q Clicked, so let's stop") 

widget.bind('a', hello)
widget.bind('q', quit)

Progress Bar

from tkinter.ttk import Progressbar

bar = Progressbar(root, length=400)
bar.grid(row=15, column=1, rowspan=1, columnspan=9)     # Progress Bar

bar['value'] = 70

Cursors

• Over Buttons:

from tkinter import *
import tkinter

top = Tk()

B1 = Button(top, text = "circle", relief = RAISED, cursor = "circle")
B2 = Button(top, text = "plus", relief = RAISED, cursor = "plus")
B1.pack()
B2.pack()
top.mainloop()

• For Root Window when busy:

def long_func():
    root.config(cursor="watch")
    root.update()
    time.sleep(30)
    root.config(cursor="")

MessageBox

• To show a message box using Tkinter, you can use messagebox library like this:

 from tkinter import messagebox
 messagebox.showinfo('Message title','Message content')

• Let’s show a message box when the user clicks a button.

from tkinter import *
from tkinter import messagebox

window = Tk()
window.title("Welcome to LikeGeeks app")
window.geometry('350x200')
def clicked():
    messagebox.showinfo('Message title', 'Message content')
btn = Button(window,text='Click here', command=clicked)

btn.grid(column=0,row=0)
window.mainloop()

Error Messages

You can show a warning message or error message with this code:

 messagebox.showwarning('Message title', 'Message content')  #shows warning message
 messagebox.showerror('Message title', 'Message content')    #shows error message

Show askquestion dialogs:

from tkinter import messagebox
res = messagebox.askquestion('Message title','Message content')
res = messagebox.askyesno('Message title','Message content')
res = messagebox.askyesnocancel('Message title','Message content')
res = messagebox.askokcancel('Message title','Message content')
res = messagebox.askretrycancel('Message title','Message content')

SpinBox

To create a Spinbox widget, you can use Spinbox class like this:

spin = Spinbox(window, from_=0, to=100)
spin = Spinbox(window, from_=0, to=100, width=5)

Complete example:

from tkinter import *
window = Tk()
window.title("Welcome to LikeGeeks app")
window.geometry('350x200')
spin = Spinbox(window, from_=0, to=100, width=5)
spin.grid(column=0,row=0)
window.mainloop()

Menu Bar

Simplify this section

To add a menu bar, you can use menu class like this:

from tkinter import Menu
menu = Menu(window)
menu.add_command(label='File')
window.config(menu=menu)

First, we create a menu, then we add our first label, and finally, we assign the menu to our window. You can add menu items under any menu by using add_cascade() function like this:

menu.add_cascade(label='File', menu=new_item)

So our code will be like this:

from tkinter import *
from tkinter import Menu
 
window = Tk()
window.title("Welcome to LikeGeeks app")

menu = Menu(window)
new_item = Menu(menu)
new_item.add_command(label='New')
menu.add_cascade(label='File', menu=new_item)

window.config(menu=menu)
window.mainloop()

Using this way, you can add many menu items as you want:

from tkinter import *
from tkinter import Menu

window = Tk()
window.title("Welcome to LikeGeeks app")

menu = Menu(window)
new_item = Menu(menu)
new_item.add_command(label='New')
new_item.add_separator()
new_item.add_command(label='Edit')
menu.add_cascade(label='File', menu=new_item)

window.config(menu=menu)
window.mainloop()

Here we add another menu item called Edit with a menu separator. You may notice a dashed line at the beginning, well, if you click that line, it will show the menu items in a small separate window. You can disable this feature by disabling the tearoff feature like this:

new_item = Menu(menu, tearoff=0)

Just replace the new_item in the above example with this one and it won’t show the dashed line anymore. I don’t need to remind you that you can type any code that works when the user clicks on any menu item by specifying the command property.

new_item.add_command(label='New', command=clicked)

Notebook widget (tab control)

To create a tab control follow these steps:

1. First, we create a tab control using Notebook class
2. Create a tab using Frame class.
3. Add that tab to the tab control.
4. Pack the tab control so it becomes visible in the window.

from tkinter import *
from tkinter import ttk

window = Tk()
window.title("This is a test App")
tab_control = ttk.Notebook(window)
tab1 = ttk.Frame(tab_control)
tab_control.add(tab1, text='First')
tab_control.pack(expand=1, fill='both')

window.mainloop()

Add widgets to Notebooks

After creating tabs, you can put widgets inside these tabs by assigning the parent property to the desired tab.

from tkinter import * from tkinter import ttk

window = Tk() window.title("Welcome to LikeGeeks app")

tab_control = ttk.Notebook(window) tab1 = ttk.Frame(tab_control) tab2 = ttk.Frame(tab_control) tab_control.add(tab1, text='First') tab_control.add(tab2, text='Second')

lbl1 = Label(tab1, text= 'label1') lbl1.grid(column=0, row=0) lbl2 = Label(tab2, text= 'label2') lbl2.grid(column=0, row=0)

tab_control.pack(expand=1, fill='both') window.mainloop()

References

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