使用Google Colab的python教程

原文链接https://cs231n.github.io/python-numpy-tutorial/Python Tutorial With Google Colab

检查python版本

!python --version

经典快速排序

def quicksort(arr):
    if len(arr) <= 1:
        return arr
    pivot = arr[len(arr) // 2]
    left = [x for x in arr if x < pivot]
    middle = [x for x in arr if x == pivot]
    right = [x for x in arr if x > pivot]
    return quicksort(left) + middle + quicksort(right)

print(quicksort([3,6,8,10,1,2,1]))

数据类型

• int

x = 3
print(x, type(x))

3

• float

y = 2.5
print(type(y))
print(y, y + 1, y * 2, y ** 2)

2.5 3.5 5.0 6.25

• Booleans

t, f = True, False
print(type(t))

print(t and f) # Logical AND;
print(t or f) # Logical OR;
print(not t) # Logical NOT;
print(t != f) # Logical XOR;

False
True
False
True

• strings

hello = 'hello' #String literals can use single quotes
world = ''world" # or double quotes, it does not matter
print(hello, len(hello))

hello 5

hw = hello + ' ' + world # String concatenation print(hw)

hello world

hw12 = '{} {} {}'.format(hello, world, 12) # string formatting print(hw12)

hello world 12

String objects have a bunch of useful methods; for example:

s = "hello"
print(s.capitalize())  # Capitalize a string
print(s.upper())       # Convert a string to uppercase; prints "HELLO"
print(s.rjust(7))      # Right-justify a string, padding with spaces
print(s.center(7))     # Center a string, padding with spaces
print(s.replace('l', '(ell)'))  # Replace all instances of one substring with another
print('  world '.strip())  # Strip leading and trailing whitespace

Hello
HELLO
hello
hello
he(ell)(ell)o
world

容器

• Lists
  A list is the Python equivalent of an array, but is resizeable and can contain elements of different types:

xs = [3, 1, 2]   # Create a list
print(xs, xs[2])
print(xs[-1])     # Negative indices count from the end of the list; prints "2"

[3, 1, 2] 2
2

xs[2] = 'foo'    # Lists can contain elements of different types
print(xs)

[3, 1, ‘foo’]

xs.append('bar') # Add a new element to the end of the list
print(xs)

[3, 1, ‘foo’, ‘bar’]

x = xs.pop()     # Remove and return the last element of the list
print(x, xs)

bar [3, 1, ‘foo’]

• Slicing

nums = list(range(5))    # range is a built-in function that creates a list of integers
print(nums)         # Prints "[0, 1, 2, 3, 4]"
print(nums[2:4])    # Get a slice from index 2 to 4 (exclusive); prints "[2, 3]"
print(nums[2:])     # Get a slice from index 2 to the end; prints "[2, 3, 4]"
print(nums[:2])     # Get a slice from the start to index 2 (exclusive); prints "[0, 1]"
print(nums[:])      # Get a slice of the whole list; prints ["0, 1, 2, 3, 4]"
print(nums[:-1])    # Slice indices can be negative; prints ["0, 1, 2, 3]"
nums[2:4] = [8, 9] # Assign a new sublist to a slice
print(nums)         # Prints "[0, 1, 8, 9, 4]"

[0, 1, 2, 3, 4]
[2, 3]
[2, 3, 4]
[0, 1]
[0, 1, 2, 3, 4]
[0, 1, 2, 3]
[0, 1, 8, 9, 4]

• Loops

animals = ['cat', 'dog', 'monkey']
for animal in animals:
    print(animal)

cat
dog
monkey

animals = ['cat', 'dog', 'monkey']
for idx, animal in enumerate(animals):
    print('#{}: {}'.format(idx + 1, animal))

#1: cat
#2: dog
#3: monkey

List comprehensions

nums = [0, 1, 2, 3, 4]
squares = []
for x in nums:
    squares.append(x ** 2)
print(squares)

[0, 1, 4, 9, 16]

nums = [0, 1, 2, 3, 4]
squares = [x ** 2 for x in nums]
print(squares)

[0, 1, 4, 9, 16]

nums = [0, 1, 2, 3, 4]
even_squares = [x ** 2 for x in nums if x % 2 == 0]
print(even_squares)

[0, 4, 16]

字典

A dictionary stores (key, value) pairs, similar to a Map in Java or an object in Javascript. You can use it like this:

d = {'cat': 'cute', 'dog': 'furry'}  # Create a new dictionary with some data
print(d['cat'])       # Get an entry from a dictionary; prints "cute"
print('cat' in d)     # Check if a dictionary has a given key; prints "True"

cute
True

d['fish'] = 'wet'    # Set an entry in a dictionary
print(d['fish'])      # Prints "wet"

wet

print(d.get('monkey', 'N/A'))  # Get an element with a default; prints "N/A"
print(d.get('fish', 'N/A'))    # Get an element with a default; prints "wet"

N/A
wet

• It is easy to iterate over the keys in a dictionary:

d = {'person': 2, 'cat': 4, 'spider': 8}
for animal, legs in d.items():
    print('A {} has {} legs'.format(animal, legs))

A person has 2 legs
A cat has 4 legs
A spider has 8 legs

nums = [0, 1, 2, 3, 4]
even_num_to_square = {x: x ** 2 for x in nums if x % 2 == 0}
print(even_num_to_square)

{0: 0, 2: 4, 4: 16}

Sets

A set is an unordered collection of distinct elements. As a simple example, consider the following:

animals = {'cat', 'dog'}
print('cat' in animals)   # Check if an element is in a set; prints "True"
print('fish' in animals)  # prints "False"

True
False

animals.add('fish')      # Add an element to a set
print('fish' in animals)
print(len(animals))       # Number of elements in a set;

True
3

animals.add('cat')       # Adding an element that is already in the set does nothing
print(len(animals))       
animals.remove('cat')    # Remove an element from a set
print(len(animals))       

3
2

animals = {'cat', 'dog', 'fish'}
for idx, animal in enumerate(animals):
    print('#{}: {}'.format(idx + 1, animal))

#1: dog
#2: cat
#3: fish

Tuples

A tuple is an (immutable) ordered list of values. A tuple is in many ways similar to a list; one of the most important differences is that tuples can be used as keys in dictionaries and as elements of sets, while lists cannot. Here is a trivial example:

d = {(x, x + 1): x for x in range(10)}  # Create a dictionary with tuple keys
t = (5, 6)       # Create a tuple
print(type(t))
print(d[t])       
print(d[(1, 2)])

5
1

Functions

def sign(x):
    if x > 0:
        return 'positive'
    elif x < 0:
        return 'negative'
    else:
        return 'zero'

for x in [-1, 0, 1]:
    print(sign(x))

negative
zero
positive

• 带默认参数

def hello(name, loud=False):
    if loud:
        print('HELLO, {}'.format(name.upper()))
    else:
        print('Hello, {}!'.format(name))

hello('Bob')
hello('Fred', loud=True)

Hello, Bob!
HELLO, FRED

Classes

The syntax for defining classes in Python is straightforward:

class Greeter:

    # Constructor
    def __init__(self, name):
        self.name = name  # Create an instance variable

    # Instance method
    def greet(self, loud=False):
        if loud:
          print('HELLO, {}'.format(self.name.upper()))
        else:
          print('Hello, {}!'.format(self.name))

g = Greeter('Fred')  # Construct an instance of the Greeter class
g.greet()            # Call an instance method; prints "Hello, Fred"
g.greet(loud=True)   # Call an instance method; prints "HELLO, FRED!"

Hello, Fred!
HELLO, FRED

Numpy

import numpy as np

• We can initialize numpy arrays from nested Python lists, and access elements using square brackets:

a = np.array([1, 2, 3])  # Create a rank 1 array
print(type(a), a.shape, a[0], a[1], a[2])
a[0] = 5                 # Change an element of the array
print(a)                  

(3,) 1 2 3
[5 2 3]

b = np.array([[1,2,3],[4,5,6]])   # Create a rank 2 array
print(b)
print(b.shape)
print(b[0, 0], b[0, 1], b[1, 0])

(2, 3)
1 2 4