Hi ok, here are my own notes on the basics of Python. I am working on a Udemy course on data science and got excited about Python(it has been while since I’ve done something cool with Python, so I am excited 🙂 ). This is based on the Udemy course https://www.udemy.com/data-science-and-machine-learning-with-python-hands-on
Where it goes, hope it helps.
You can run python code as scripts or as a python notebook.
Running a script from a command prompt:
python “your script file location and name”
Basic basics
List definition:
listOfNumbers = [1, 2, 3, 4, 5, 6]
Iteration through a list of items:
for number in listOfNumbers:
print number,
if (number % 2 == 0):
print “is even”
else:
print “is odd”
#Notice: In python you differentiate between blocks of code by whitespace, or a tab. Not the same as let say Java or C# where the char { and } are used to differentiate a block of code. Pay attention to you formatting, indentation.
#Notice: The char , (comma) is used to tell that something is going to continue on the same print line, within the same block of code. See example above.
#Notice: Colons : differentiate clauses.
Importing modules
import numpy as np
A = np.random.normal(25.0, 5.0, 10)
print A
Data structures
Lists
Defining a list(Notice: The brackets [] define an mutable list):
x = [1, 2, 3, 4, 5, 6]
Printing the length of a list:
print len(x)
Sub setting lists:
First 3 elements(counting starts from zero):
x[:3]
Last 3 elements:
x[3:]
Last two elements from starting from the end of the list:
x[-2:]
Extend the list with a new list :
x.extend([7,8])
Add a new item to the list:
x.append(9)
Python is a weekly typed language which allows you to put whatever you want in a list:
Creating a multidimensional list:
y = [10, 11, 12]
listOfLists = [x, y]
listOfLists
Sort a list(descending):
z = [3, 2, 1]
z.sort()
Sort a list(ascending):
z.sort(reverse=True)
Tuples
Are just like lists but immutable.
You can not extend them append them, sort them etc. You can not change them.
Example:
#Tuples are just immutable lists. Use () instead of []
x = (1, 2, 3)
len(x)
y = (4, 5, 6)
listOfTuples = [x, y]
Tuples common usage for data science or data processing:
Is to use it to assign variables to input data that as it is read in.
This example creates variable with values from a “source” where data is split by the comma.
#Notice: It is important that you have the same about of variables in your tuple as you are retrieving/assigning from the data “source”.
(age, income) = “32,120000”.split(‘,’)
print age
print income
Dictionaries
A way to define a “lookup” table:
# Like a map or hash table in other languages
captains = {}
captains[“Enterprise”] = “Kirk”
captains[“Enterprise D”] = “Picard”
captains[“Deep Space Nine”] = “Sisko”
captains[“Voyager”] = “Janeway”
print captains[“Voyager”]
print captains.get(“Enterprise”)
for ship in captains:
print ship + “: ” + captains[ship]
If something is not found the result will be none:
print captains.get(“NX-01”)
Functions
Let you repeat a set of operation over and over again with different parameters.
Notice: use def to define a function and () chars to define the parameters and use the return keyword to return value from the function.
def SquareIt(x):
return x * x
print SquareIt(2)
Pass functions around as parameters
#Notice: You have to make sure that what you are typing is correct because there is no strong typing in Python. Typing the wrong function name will cause errors.
#You can pass functions around as parameters
def DoSomething(f, x):
return f(x)
print DoSomething(SquareIt, 3)
Lambda functions
Is Functional programming: You can inline a function into a function
#Notice: lambda keyword is telling that you are defining a inline function to be used where you put it. In the example below inside a function parameter named x followed by a colon : character followed by what the function actually does. To pass in multiple parameters to a lambda function use the comma , to separate the variables.
#Lambda functions let you inline simple functions
print DoSomething(lambda x: x * x * x, 3)
Boolean Expressions
Valye is false
print 1 == 3
Value is true(or keyword check which is true)
print (True or False)
Check if something is a certain value( Use the is keyword)
print 1 is 3
If else clauses
if 1 is 3:
print “How did that happen?”
elif 1 > 3:
print “Yikes”
else:
print “All is well with the world”
Looping
Normal looping
for x in range(10):
print x,
Continue and Break
#Process only the first 5 items but skip the number one
for x in range(10):
if (x is 1):
continue
if (x > 5):
break
print x,
While
x = 0
while (x < 10):
print x,
x += 1
