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:
- Unlike append, len is a normal function rather than a method.
- 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
- All keys are unique, If you update item with same keys, Value will get replaced.
- 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
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)
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")
Examples
Entering variables inside quotation marks:
os.system("""osascript -e 'display notification "{}" with title "{}" sound name "{}"'""".format(text, title, sound))
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')
Common Packages
paramiko SSH ipaddr IP addressing
Slack
def slack(message, channel, icon):
sc = SlackClient("xoxp-353395......6a42822")
sc.api_call('chat.postMessage', channel=channel, text=message, username='SREBot', icon_emoji=icon)
slack("Hello", "@aman", ':scroll:')
slack("Hello", "team-alerts", ':scroll:')
Salesforce
Installation
pip3 install simple_salesforce
Find out Security Token:
https://<yoursalesforcehostname>/_ui/system/security/ResetApiTokenEdit?retURL=%2Fui%2Fsetup%2FSetup%3Fsetupid%3DPersonalInfo&setupid=ResetApiToken
Example:
https://avi.my.salesforce.com/_ui/system/security/ResetApiTokenEdit?retURL=%2Fui%2Fsetup%2FSetup%3Fsetupid%3DPersonalInfo&setupid=ResetApiToken
Login:
sf = Salesforce(username="test", password="pwd@123", security_token="NSUgqlxewweWdK8bjBEsdsNs")
Query:
results = sf.query_all("SELECT CaseNumber,Subject,IsClosed FROM Case WHERE OwnerId = '00500000erefBz'")
result2 = sf.query_all("SELECT CaseNumber,Subject,IsClosed FROM Case WHERE CaseNumber = '6800'")
Output:
results['records'])
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
Traceback
import traceback
try:
<something>
except:
traceback.print_exc()
pass
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)
Misc
- Python 2 to Python 3 conversion tool
2to3 -w test.py
- References
{{#widget:DISQUS
|id=networkm
|uniqid=Python
|url=https://aman.awiki.org/wiki/Python
}}