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407 lines
12 KiB
407 lines
12 KiB
# Python中单行注释用#表示，#之后同行字符全部认为被注释
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""" 与之对应的是多行注释
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用三个双引号表示，这两段双引号当中的内容都会被视作是注释
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"""
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values = []
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kv = {'hello': 'world'}
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# 获得一个整数
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values[0] = 3
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# 获得一个浮点数
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values[1] = 10.0
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c = 1 + 1 # => 2
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d = 8 - 1 # => 7
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e = 10 * 2 # => 20
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f = 35 / 5 # => 7.0
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g = 5 // 3 # => 1
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h = -5 // 3 # => -2
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j = 5.5 // 3.0 # => 1.0 # works on floats too
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k = -5.0 // 3.0 # => -2.0
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# Modulo operation
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values[10] = 7 % 3 # => 1
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# Exponentiation (x**y, x to the yth power)
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values[11] = 2 ** 3 # => 8
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# Enforce precedence with parentheses
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values[12] = 1 + 3 * 2 # => 7
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values[13] = (1 + 3) * 2 # => 8
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_ = True # => True
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_ = False # => False
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_ = not True # => False
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_ = not False # => True
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# Boolean Operators
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# Note "and" and "or" are case-sensitive
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_ = True and False # => False
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_ = False or True # => True
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_ = True + True # => 2
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_ = True * 8 # => 8
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_ = False - 5 # => -5
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_ = 0 == False # => True
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_ = 1 == True # => True
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_ = 2 == True # => False
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_ = -5 != False # => True
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_ = bool(0) # => False
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_ = bool(4) # => True
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_ = bool(-6) # => True
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_ = 0 and 2 # => 0
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_ = -5 or 0 # => -5
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# Equality is ==
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_ = 1 == 1 # => True
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_ = 2 == 1 # => False
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# Inequality is !=
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_ = 1 != 1 # => False
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_ = 2 != 1 # => True
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# More comparisons
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_ = 1 < 10 # => True
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_ = 1 > 10 # => False
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_ = 2 <= 2 # => True
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_ = 2 >= 2 # => True
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# Seeing whether a value is in a range
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_ = 1 < 2 and 2 < 3 # => True
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_ = 2 < 3 and 3 < 2 # => False
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# Chaining makes this look nicer
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_ = 1 < 2 < 3 # => True
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_ = 2 < 3 < 2 # => False
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a = [1, 2, 3, 4] # Point a at a new list, [1, 2, 3, 4]
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b = a # Point b at what a is pointing to
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_ = b is a # => True, a and b refer to the same object
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_ = b == a # => True, a's and b's objects are equal
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_ = b = [1, 2, 3, 4] # Point b at a new list, [1, 2, 3, 4]
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_ = b is a # => False, a and b do not refer to the same object
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_ = b == a # => True, a's and b's objects are equal
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# Strings are created with " or '
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_ = "This is a string."
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_ = 'This is also a string.'
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# Strings can be added too! But try not to do this.
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_ = "Hello " + "world!" # => "Hello world!"
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# String literals (but not variables) can be concatenated without using '+'
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_ = "Hello " "world!" # => "Hello world!"
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# A string can be treated like a list of characters
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_ = "This is a string"[0] # => 'T'
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# You can find the length of a string
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_ = len("This is a string") # => 16
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# You can also format using f-strings or formatted string literals (in Python 3.6+)
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name = "Reiko"
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_ = f"She said her name is {name}." # => "She said her name is Reiko"
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# You can basically put any Python statement inside the braces and it will be output in the string.
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_ = f"{name} is {len(name)} characters long." # => "Reiko is 5 characters long."
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# None is an object
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_ = None # => None
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# Don't use the equality "==" symbol to compare objects to None
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# Use "is" instead. This checks for equality of object identity.
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_ = "etc" is None # => False
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_ = None is None # => True
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# None, 0, and empty strings/lists/dicts/tuples all evaluate to False.
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# All other values are True
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_ = bool(None) # => False
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_ = bool(0) # => False
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_ = bool("") # => False
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_ = bool([]) # => False
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_ = bool({}) # => False
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_ = bool(()) # => False
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# Python has a print function
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print("I'm Python. Nice to meet you!") # => I'm Python. Nice to meet you!
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# By default the print function also prints out a newline at the end.
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# Use the optional argument end to change the end string.
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print("Hello, World", end="!") # => Hello, World!
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# Simple way to get input data from console
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input_string_var = input("Enter some data: ") # Returns the data as a string
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# Note: In earlier versions of Python, input() method was named as raw_input()
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# There are no declarations, only assignments.
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# Convention is to use lower_case_with_underscores
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some_var = 5
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# Accessing a previously unassigned variable is an exception.
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# See Control Flow to learn more about exception handling.
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# if can be used as an expression
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# Equivalent of C's '?:' ternary operator
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_ = "yahoo!" if 3 > 2 else 2 # => "yahoo!"
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def test():
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if 3 > 2:
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return 'yahoo'
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else:
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return 2
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# Lists store sequences
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li = []
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# You can start with a prefilled list
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other_li = [4, 5, 6]
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# Add stuff to the end of a list with append
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li.append(1) # li is now [1]
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li.append(2) # li is now [1, 2]
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li.append(4) # li is now [1, 2, 4]
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li.append(3) # li is now [1, 2, 4, 3]
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# Remove from the end with pop
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li.pop() # => 3 and li is now [1, 2, 4]
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# Let's put it back
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li.append(3) # li is now [1, 2, 4, 3] again.
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# Access a list like you would any array
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_ = li[0] # => 1
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# Look at the last element
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_ = li[-1] # => 3
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# Looking out of bounds is an IndexError
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_ = li[4] # Raises an IndexError
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# You can look at ranges with slice syntax.
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# The start index is included, the end index is not
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# (It's a closed/open range for you mathy types.)
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_ = li[1:3] # Return list from index 1 to 3 => [2, 4]
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_ = li[2:] # Return list starting from index 2 => [4, 3]
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_ = li[:3] # Return list from beginning until index 3 => [1, 2, 4]
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_ = li[::2] # Return list selecting every second entry => [1, 4]
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_ = li[::-1] # Return list in reverse order => [3, 4, 2, 1]
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# Use any combination of these to make advanced slices
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# li[start:end:step]
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# Make a one layer deep copy using slices
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li2 = li[:] # => li2 = [1, 2, 4, 3] but (li2 is li) will result in false.
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# Remove arbitrary elements from a list with "del"
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del li[2] # li is now [1, 2, 3]
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# Remove first occurrence of a value
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li.remove(2) # li is now [1, 3]
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li.remove(2) # Raises a ValueError as 2 is not in the list
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# Insert an element at a specific index
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li.insert(1, 2) # li is now [1, 2, 3] again
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# Get the index of the first item found matching the argument
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li.index(2) # => 1
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li.index(4) # Raises a ValueError as 4 is not in the list
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# Tuples are like lists but are immutable.
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tup = (1, 2, 3)
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tup[0] # => 1
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tup[0] = 3 # Raises a TypeError
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type((1)) # => <class 'int'>
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type((1,)) # => <class 'tuple'>
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type(()) # => <class 'tuple'>
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_ = len(tup) # => 3
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_ = tup + (4, 5, 6) # => (1, 2, 3, 4, 5, 6)
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_ = tup[:2] # => (1, 2)
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_ = 2 in tup # => True
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# You can unpack tuples (or lists) into variables
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a, b, c = (1, 2, 3) # a is now 1, b is now 2 and c is now 3
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# You can also do extended unpacking
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# Tuples are created by default if you leave out the parentheses
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d, e, f = 4, 5, 6 # tuple 4, 5, 6 is unpacked into variables d, e and f
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# respectively such that d = 4, e = 5 and f = 6
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# Now look how easy it is to swap two values
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e, d = d, e # d is now 5 and e is now 4
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# Look up values with []
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invalid_dict = {1: "123"}
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_ = invalid_dict["one"] # => 1
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_ = invalid_dict.get('one') # => 1
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# Here is a prefilled dictionary
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filled_dict = {"one": 1, "two": 2, "three": 3}
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# Get all keys as an iterable with "keys()". We need to wrap the call in list()
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# to turn it into a list. We'll talk about those later. Note - for Python
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# versions <3.7, dictionary key ordering is not guaranteed. Your results might
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# not match the example below exactly. However, as of Python 3.7, dictionary
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# items maintain the order at which they are inserted into the dictionary.
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_ = list(filled_dict.keys()) # => ["three", "two", "one"] in Python <3.7
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_ = list(filled_dict.keys()) # => ["one", "two", "three"] in Python 3.7+
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# Get all values as an iterable with "values()". Once again we need to wrap it
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# in list() to get it out of the iterable. Note - Same as above regarding key
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# ordering.
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_ = list(filled_dict.values()) # => [3, 2, 1] in Python <3.7
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_ = list(filled_dict.values()) # => [1, 2, 3] in Python 3.7+
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# Check for existence of keys in a dictionary with "in"
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_ = "one" in filled_dict # => True
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_ = 1 in filled_dict # => False
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# _ = {'a': 1, **{'b': 2}} # => {'a': 1, 'b': 2}
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# _ = {'a': 1, **{'a': 2}} # => {'a': 2}
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# Sets store ... well sets
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empty_set = set()
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# Initialize a set with a bunch of values. Yeah, it looks a bit like a dict. Sorry.
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some_set = {1, 1, 2, 2, 3, 4} # some_set is now {1, 2, 3, 4}
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# Do set intersection with &
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# 计算交集
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other_set = {3, 4, 5, 6}
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filled_set = {1, 2, 3}
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_ = filled_set & other_set # => {3, 4, 5}
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# Do set union with |
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# 计算并集
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_ = filled_set | other_set # => {1, 2, 3, 4, 5, 6}
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# Do set difference with -
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# 计算差集
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_ = {1, 2, 3, 4} - {2, 3, 5} # => {1, 4}
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# Do set symmetric difference with ^
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# 这个有点特殊，计算对称集，也就是去掉重复元素剩下的内容
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_ = {1, 2, 3, 4} ^ {2, 3, 5} # => {1, 4, 5}
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# Check if set on the left is a superset of set on the right
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_ = {1, 2} >= {1, 2, 3} # => False
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# Check if set on the left is a subset of set on the right
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_ = {1, 2} <= {1, 2, 3} # => True
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if some_var > 10:
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print("some_var is totally bigger than 10.")
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elif some_var < 10: # This elif clause is optional.
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print("some_var is smaller than 10.")
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else: # This is optional too.
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print("some_var is indeed 10.")
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for animal in ["dog", "cat", "mouse"]:
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# You can use format() to interpolate formatted strings
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print("{} is a mammal".format(animal))
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for i in range(4):
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print(i)
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animals = ["dog", "cat", "mouse"]
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for i, value in enumerate(animals):
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print(i, value)
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x = 0
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while x < 4:
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print(x)
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x += 1 # Shorthand for x = x + 1
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# Handle exceptions with a try/except block
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try:
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# Use "raise" to raise an error
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raise IndexError("This is an index error")
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except IndexError as e:
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pass # Pass is just a no-op. Usually you would do recovery here.
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except (TypeError, NameError):
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pass # Multiple exceptions can be handled together, if required.
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finally: # Execute under all circumstances
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print("We can clean up resources here")
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# Instead of try/finally to cleanup resources you can use a with statement
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# 代替使用try/finally语句来关闭资源
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with open("myfile.txt") as f:
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for line in f:
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print(line)
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# Writing to a file
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# 使用with写入文件
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contents = {"aa": 12, "bb": 21}
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with open("myfile1.txt", "w+") as file:
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file.write(str(contents)) # writes a string to a file
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with open("myfile2.txt", "w+") as file:
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file.write(json.dumps(contents)) # writes an object to a file
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# Reading from a file
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# 使用with读取文件
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with open('myfile1.txt', "r+") as file:
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contents = file.read() # reads a string from a file
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print(contents)
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# print: {"aa": 12, "bb": 21}
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with open('myfile2.txt', "r+") as file:
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contents = json.load(file) # reads a json object from a file
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print(contents)
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# print: {"aa": 12, "bb": 21}
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# Python offers a fundamental abstraction called the Iterable.
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# An iterable is an object that can be treated as a sequence.
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# The object returned by the range function, is an iterable.
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filled_dict = {"one": 1, "two": 2, "three": 3}
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our_iterable = filled_dict.keys()
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print(our_iterable) # => dict_keys(['one', 'two', 'three']). This is an object that implements our Iterable interface.
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# We can loop over it.
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for i in our_iterable:
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print(i) # Prints one, two, three
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# However we cannot address elements by index.
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our_iterable[1] # Raises a TypeError
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# An iterable is an object that knows how to create an iterator.
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our_iterator = iter(our_iterable)
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# Our iterator is an object that can remember the state as we traverse through it.
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# We get the next object with "next()".
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next(our_iterator) # => "one"
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# It maintains state as we iterate.
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next(our_iterator) # => "two"
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next(our_iterator) # => "three"
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# After the iterator has returned all of its data, it raises a StopIteration exception
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next(our_iterator) # Raises StopIteration
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# We can also loop over it, in fact, "for" does this implicitly!
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our_iterator = iter(our_iterable)
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for i in our_iterator:
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print(i) # Prints one, two, three
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# You can grab all the elements of an iterable or iterator by calling list() on it.
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list(our_iterable) # => Returns ["one", "two", "three"]
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list(our_iterator) # => Returns [] because state is saved
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# Use "def" to create new functions
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def add(x, y):
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print("x is {} and y is {}".format(x, y))
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return x + y # Return values with a return statement
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# Calling functions with parameters
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add(5, 6) # => prints out "x is 5 and y is 6" and returns 11
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# Another way to call functions is with keyword arguments
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add(y=6, x=5) # Keyword arguments can arrive in any order.
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# You can define functions that take a variable number of
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# positional arguments
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def varargs(*args):
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return args
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varargs(1, 2, 3) # => (1, 2, 3)
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