• Python stores large matrix in a file with HDF5 format
  • Store video in file Matrix video

• Science Python SciPy

  # install scipy on my MacOS
  python3 -m pip install --user numpy scipy matplotlib ipython jupyter pandas sympy nose
  

• Numerical computation with numpy NumPy
• Convert string to int or float package: fastnumbers
• Plot and statistics package: matplotlib
• Iterator, itertools, fast loop : itertools
  • speed up your loop

• Python English dictionary words. pyenchant

  • StackOverFlow SO

  python3 -m pip install pyenchant
  

• Browser Editor Python Jupyter Lab

  python3 -m pip install jupyterlab
  

• Python has class, it is similar to Java and C++, but it is more flexible.

  • Python DOES not have access modifier such as public, protected and private

  class Person:
  name
          __init__(self, name):
          self.name = name
  
  c = Person("David")
  print(c.name)
  

  • Python __call__ and __init__

    class Person:
                    __init__(self, name):
                    self.name = name
                    __call__(self, newName):
                    self.newName = newName
    

    The difference is __call__ is used to instantiate object, but __call__ is used to modify and instance.

    p = Person("David")
    print(p.name) # => David
            p1 = p("Michael") 
            print(p.name) # => Michael
    

    • It is almost like setter and getter in Java

  • Why Python is weird

    • self in Python is not a keyeword. WOW, so you can do the following.
    • If you are an Java programmer, you might like it.

    class Thing:
       name = 'David'
       def __init__(this, name):
              this.name = name
            // no problem
    

  • Python needs less code to create a class comparing to Java, C# or C++
    • Java class is more complicated.

   public class Person{
  public String name;
  public Person(String name){
    this.name = name;
  }
   }
  Person p = new Person("David");
  System.out.print(p.name);
  

• Use following import statement does not work in Python console or source file.

          # wrong 
  import AronLib 
          # right
          import AronLib as a
  

• one underscore means private variable.

class Person:
_name
__init__(self, name)
        self._name = name

• Define a class variable called trick for a dog.

class Dog:
   trick = []
   def __init__(self, name):
  self.name = name
   def append(self, trick)
  self.trick.append(trick)

# trick will be shared for all instance Dog
d1 = Dog('Fido')
d2 = Dog('xFido')
d1.append('new trick')
d2.append('old trick')
print(d2.trick) # ['new trick', 'old trick']

• Never use class variable in Python

class Dog:
  def __init__(self, name):
 self.trick = []
 self.name = name
  def append(self, trick):
 self.trick.append(trick)

d1 = Dog('Fido')
d2 = Dog('xFido')
d1.append('new trick')
d2.append('old trick')
print(d2) # ['old trick']

• Python uses simple way to initialize a list, it is like Haskell.

ls = [1, 2, 3]                                                             
ls = ['dog', 'cat', 'cow']                                                 

• In Haskell

let ls = [1, 2, 3]                                                         
let ls = ["dog", "cat", "cow"]                                             

• In Java, you need lots more code.

List<Integer> ls = new ArrayList<>();                                          
ls.add(1);                                                                 
ls.add(2);                                                                 
ls.add(3);                                                                 

// Better way to initialize List                                           
List<Integer> ls = Arrays.asList(1, 2, 3);                                 

• In C++, it is relative easy.

std::vector<int> vec = {1, 2, 3};                                          
std::vector<string> vec = {"dog", "cat", "cow"};                           

• Python takes some nice features from Haskell.

  ls = [ x for x in [1, 2, 3] if x % == 0]
  # ls = [2]
  

• Nested list comprehension.

  lss = [[x for x in [1, 2, 3] for y in [1, 1, 1]]
  # lss = [[1, 2, 3], [1, 2, 3], [1, 2, 3]]
  

• Python does not have loop like C++/Java, but it has range and it is better.

items = 'abc'
# 0 <= i < 10
for i in len(items):
        print(items[i])

• Nested list is very easy in Python

  s1 = [1, 2, 3]
  s2 = ['dog', 'cat']
  s3 = [s1, s2]
  print(s3) # [[1, 2, 3], ['dog', 'cat']]
  

• In Haskell, nested list is similar Python but it is only for the same type.

  • This is main difference between static and dynamic type.

  let s1 = [1, 2, 3]
  let s2 = [11, 22]
  let s3 = [s1, s2] 
  --  s3 = [[1, 2, 3], [11, 22]]
  

• Also, you need to understand List is a interface and ArrayList is a class in Java.
• Python is a programming language that you can get stuff done now.
• Let's sort a list that contains many tuples.

import AronLib as A
ls = [(1, "dog"), (2, "cow"), (0, "cat")]
ls.sort(key=lambda x: x[0])
print(ls)

• It is very simple code to solve it.

• If do the same think in Java, then you will need lots more code.

  // add java code to solve above problem.
  // There is no tuple in Java originally, 
  // define a class instead
  // In the old Java, you need to implement Comparable or Comparator
  // But the new Java, you can use lambda to get away it with Collections.sort()
  

ls1 = [1, 2, 3]
ls2 = [11, 22]
ls3 = ls1 + ls2

• In Java, you can us addAll method from the list. It is not very elegrant, but it works.

  • I wish I can overload the \( + \) operator for list, unfortunately Java does not allow operator overloading.

  List<Integer> ls1 = Arrays.asList(1, 2);
  List<Integer> ls2 = Arrays.asList(11, 22);
  ls1.addAll(ls2);
  // ls1 = [1, 2, 11, 22]
  

• In C++, it is less elegrant if you need to concate two vectors.(We use vector instead of list)
  • list and vector are both STL template container.
• list is implemented in Double Linkedlist, it is non-cotiguous memory.

• vector is implemented in array, it is cotiguous memory.

  

• Here is C++ code to concate two vectors.

  • \( + \) can be overloaded for two vectors if you want to.

  vector<int> v1 = {1, 2, 3};
  vector<int> v2 = {11, 22};
      v1.insert(v1.end(), v2.begin(), v2.end());
      // v2 = {1, 2, 3, 11, 22}
  

ls = [1, 2, 3, 4, 5]
ss = ls[-2:]
# ss = [4, 5]

• -2: it means take 2 elements from the left.
• 2: it means take 2 elements from the right.

ls = [1, 2, 3, 4, 5]
ss = ls[2:]
# ss = [1, 2]

• In Haskell, you can do the following

          -- take n elements from the left
  let ls = [1, 2, 3, 4, 5]
          let ss = drop (length ls - n) ls
  

• Python has lambda too. Well everyone uses lambda this day because it is COOL

  f = lambda x, y: x + y
  

• It is very close to Haskell

  let f = \x y -> x + y
  

  • In C++, lambda function can be defined as following.

    auto f = [](auto& x, auto& y) { return x + y;};
    // or 
    std::function<int(int, int)> f = [](auto& x, auto& y) { return x + y;};
    

  • In Java, lambda function is less flexible.

    Function<Integer, Boolean> f = x -> x % 2 == 0;
    BiFunction<Integer, Integer, Integer> f = (x, y) -> x + y;
        // You need to define your lambda interface if you need to pass more parameters.
    

  • Haskell is more elegant. I should not compare Python and Haskell because they are in different world completely.

• Install matplotlib

python3 -m pip install -U matplotlib

• plot \( f(x) = x^3 \)

import matplotlib.pyplot as plt

# plot f(x) = x^3
x = [0.1*x for x in range(-10, 10, 1) ]
y = [(0.1*x)**3 for x in range(-10, 10, 1) ]
plt.plot(x, y)
plt.ylabel('some numbers')
plt.show()

• Python regex expression regex Python3 Regex
• Split word with \W Regex Expression Extension

import re

sentence = 'what is that'
m = re.split(r'\W+', sentence)
print(m)
m1 = re.split(r'(\W+)', sentence)
print(m1)

• Find all integers that satify the following equation \begin{align*}

a, b, c, d & ∈ \mathbf{N}
a³ + b³ + c³ &= d³ \end{align*}

• Python code to solve it, naive way.

  n = [x for x in range(1, 101, 1)]
  for a in n:
    for b in n:
                  for c in n:
                    for d in n:
                          if d > a && d > b && d > c and a**3 + b**3 + c**3 == d**3:
                             print(a, b, c, d)
  

• Better way to solve it.

  ns = [x for x in range(1, 10)]
  tu = [
           (a, b, c, d)
           for a in ns for b in ns for c in ns for d in ns if d > a and d > b and d > c and a**3 + b**3 + c**3 == d**3
  ]
  

• Python does not have Array like other languages such as Java and C++.
  • Array means it can only contain primary date type
• Instead Python has list which is similar to Array and it can contain any object
  • list is like vector or list in C++ or ArrayList in Java.

ls1 = [1, 2, 3]
ls2 = ['dog', 'cat', 'cow', 'pig']

• Dictionary in Python
  • Python has Dictionary or HashMap or HashTable.

dict = {'dog' : 1, 'cat':2}

• Iterator through Dictionary

dict = {1:'dog', 2:'cat'}
        for k in dict:
           print(dict[k])

• The difference between Tuple and Dictionary in Python
  • Tuple is like Dictionary but all its elements are immutable.

t = (1, 2, 3)
        t[0] = 4; # error

        dict = {1 : 'dog'}
        dict[1] = 'cat' # no problem

• NumPy does have Array

  import numpy as np
  arr1 = np.array([1, 2, 3])
  ls = [x for x in range(1, 10)]
  arr2 = np.array(ls)
  
  

• Set in Python

  • The value of set is immutable in Python so it can not contain other set

  myset = set()
  myset.add(1)
  myset.add(2)
  myset.add(1)
  print(myset)
  # {1, 2}
  

  • Add set to set using frozenset in Python

s1 = set()
        s2 = set()
        s1.add(frozenset(s2))

• Check whether a varaible is string or not in Python
• When to use it?
  • When a variable is passed into a function, the function needs to know the type of variable.

def fl(var = None):
                if var is None:
                   print('cool')
                elif isinstance(var, string_types):
                   print('it is not cool')
                else:
                   print('it is awesome')

from six import string_types
str = "dog"
if isinstance(str, string_types):
   print("yes")
else:
   print("no")

• The differences between Python 2 and Python 3 Diff Python 2 and Python 3
• map return a list in Python 2

• map return a iterator in Python 3, Yes, I'm always wondering why we need following:

  # Python 3
  ls = [1, 2, 3]
  newlist = list(map(lambda x: x + 1, ls)
  

• Python 2 has int and long, Python 3 only has int

• You can code a binary search tree in less then 10 mins

• In Java, you might need an half hour.

  # Node
  class Node:
    def __init__(self, data):
           self.data = data
           self.left = None
           self.right = None
  
  # insert a node to a binary tree
  # if data < then the current data
  #    traveral to left subtree
  # else
  #    traveral to right subtree
  #
  def insert(root, node):
          if root is None:
             root = node
          else:
             if node.data < root.data:
                    if root.left == None:
                           root.left = node
                    else:
                           insert(root.left, node)
             else:
                    if root.right == None:
                           root.right = node
                    else: 
                           insert(root.right, node)
  
  

• Python code without type, you can pass anything to everything and Python interpeter never complain.

• When you run pip install package_name, you might not know which Python version the package uses
  • Ex: I install Rabbit message queue on on FreeBSD, but pip use Python 3.6.

On my MacOS, I use Python 3.7 to run all the Python scripts, and Python 3.7 can not find module pika which is Python module for Rabbitmq.

• I have many Python versions on my MacOS now. I have no idea which to which??

• When you try to conver character to int, ord() is used.

  ord('a') => 97
  chr(97) => 'a'
  

• No clue who come up the name of function??, 4) Tuple in Python

  t = (1, 2, 3)
  print(t) # => (1, 2, 3)
  t[0] # => 1
  t[1] # => 2
  t[2] # => 3
  # tuple to list
  ls = list(t)
  print(ls) # => [1, 2, 3]
  

• Code in Python Redis
• Connect to Redis Server
• Set key and value
• Retrieve a key

python3 -m pip install redis