Hướng dẫn implies python
In the following examples, input and output are distinguished by the presence or absence of prompts (>>> and …): to repeat the example, you must type everything after the prompt, when the prompt appears; lines that do not begin with a prompt are output from the interpreter. Note that a secondary prompt on a line by itself in an example means you must type a blank line; this is used to end a multi-line command.
You can toggle the display of prompts and output by clicking on
Many of the examples in this
manual, even those entered at the interactive prompt, include comments. Comments in Python start with the hash character,
# this is the first comment spam = 1 # and this is the second comment # ... and now a third! text = "# This is not a comment because it's inside quotes."
3.1. Using Python as a Calculator¶
Let’s try some simple Python commands. Start the interpreter and wait for the primary prompt,
The interpreter acts as a simple calculator: you can type an expression at it and it will write the value. Expression syntax is straightforward: the operators
>>> 2 + 2 4 >>> 50 - 5*6 20 >>> (50 - 5*6) / 4 5.0 >>> 8 / 5 # division always returns a floating point number 1.6
The integer numbers (e.g.
>>> 17 / 3 # classic division returns a float 5.666666666666667 >>> >>> 17 // 3 # floor division discards the fractional part 5 >>> 17 % 3 # the % operator returns the remainder of the division 2 >>> 5 * 3 + 2 # floored quotient * divisor + remainder 17
With Python, it is possible to use the
>>> 5 ** 2 # 5 squared 25 >>> 2 ** 7 # 2 to the power of 7 128
>>> width = 20 >>> height = 5 * 9 >>> width * height 900
If a variable is not “defined” (assigned a value), trying to use it will give you an error:
>>> n # try to access an undefined variable Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'n' is not defined
There is full support for floating point; operators with mixed type operands convert the integer operand to floating point:
In interactive mode, the last printed expression is assigned to the variable
>>> tax = 12.5 / 100 >>> price = 100.50 >>> price * tax 12.5625 >>> price + _ 113.0625 >>> round(_, 2) 113.06
This variable should be treated as read-only by the user. Don’t explicitly assign a value to it — you would create an independent local variable with the same name masking the built-in variable with its magic behavior.
In addition to
Besides numbers, Python can also manipulate strings, which
can be expressed in several ways. They can be enclosed in single quotes (
>>> 'spam eggs' # single quotes 'spam eggs' >>> 'doesn\'t' # use \' to escape the single quote... "doesn't" >>> "doesn't" # ...or use double quotes instead "doesn't" >>> '"Yes," they said.' '"Yes," they said.' >>> "\"Yes,\" they said." '"Yes," they said.' >>> '"Isn\'t," they said.' '"Isn\'t," they said.'
In the interactive interpreter, the output string is enclosed in quotes and special characters are escaped with backslashes. While this might sometimes look different from the input (the enclosing quotes
could change), the two strings are equivalent. The string is enclosed in double quotes if the string contains a single quote and no double quotes, otherwise it is enclosed in single quotes. The
>>> '"Isn\'t," they said.' '"Isn\'t," they said.' >>> print('"Isn\'t," they said.') "Isn't," they said. >>> s = 'First line.\nSecond line.' # \n means newline >>> s # without print(), \n is included in the output 'First line.\nSecond line.' >>> print(s) # with print(), \n produces a new line First line. Second line.
If you don’t want characters prefaced by
>>> print('C:\some\name') # here \n means newline! C:\some ame >>> print(r'C:\some\name') # note the r before the quote C:\some\name
String literals can span multiple lines. One way is using triple-quotes:
print("""\ Usage: thingy [OPTIONS] -h Display this usage message -H hostname Hostname to connect to """)
produces the following output (note that the initial newline is not included):
Usage: thingy [OPTIONS] -h Display this usage message -H hostname Hostname to connect to
Strings can be concatenated (glued together) with the
>>> # 3 times 'un', followed by 'ium' >>> 3 * 'un' + 'ium' 'unununium'
Two or more string literals (i.e. the ones enclosed between quotes) next to each other are automatically concatenated.
This feature is particularly useful when you want to break long strings:
>>> text = ('Put several strings within parentheses ' ... 'to have them joined together.') >>> text 'Put several strings within parentheses to have them joined together.'
This only works with two literals though, not with variables or expressions:
>>> prefix = 'Py' >>> prefix 'thon' # can't concatenate a variable and a string literal File "<stdin>", line 1 prefix 'thon' ^ SyntaxError: invalid syntax >>> ('un' * 3) 'ium' File "<stdin>", line 1 ('un' * 3) 'ium' ^ SyntaxError: invalid syntax
If you want to
concatenate variables or a variable and a literal, use
>>> prefix + 'thon' 'Python'
Strings can be indexed (subscripted), with the first character having index 0. There is no separate character type; a character is simply a string of size one:
>>> word = 'Python' >>> word # character in position 0 'P' >>> word # character in position 5 'n'
Indices may also be negative numbers, to start counting from the right:
>>> word[-1] # last character 'n' >>> word[-2] # second-last character 'o' >>> word[-6] 'P'
Note that since -0 is the same as 0, negative indices start from -1.
In addition to indexing, slicing is also supported. While indexing is used to obtain individual characters, slicing allows you to obtain substring:
>>> word[0:2] # characters from position 0 (included) to 2 (excluded) 'Py' >>> word[2:5] # characters from position 2 (included) to 5 (excluded) 'tho'
Slice indices have useful defaults; an omitted first index defaults to zero, an omitted second index defaults to the size of the string being sliced.
>>> word[:2] # character from the beginning to position 2 (excluded) 'Py' >>> word[4:] # characters from position 4 (included) to the end 'on' >>> word[-2:] # characters from the second-last (included) to the end 'on'
Note how the start is always included, and the end always excluded. This makes sure that
>>> word[:2] + word[2:] 'Python' >>> word[:4] + word[4:] 'Python'
One way to remember how slices work is to think of the indices as pointing between characters, with the left edge of the first character numbered 0. Then the right edge of the last character of a string of n characters has index n, for example:
+---+---+---+---+---+---+ | P | y | t | h | o | n | +---+---+---+---+---+---+ 0 1 2 3 4 5 6 -6 -5 -4 -3 -2 -1
The first row of numbers gives the position of the indices 0…6 in the string; the second row gives the corresponding negative indices. The slice from i to j consists of all characters between the edges labeled i and j, respectively.
For non-negative indices, the length of a slice is the difference of the indices, if both are within bounds. For example, the length of
Attempting to use an index that is too large will result in an error:
>>> word # the word only has 6 characters Traceback (most recent call last): File "<stdin>", line 1, in <module> IndexError: string index out of range
However, out of range slice indexes are handled gracefully when used for slicing:
>>> word[4:42] 'on' >>> word[42:] ''
Python strings cannot be changed — they are immutable. Therefore, assigning to an indexed position in the string results in an error:
>>> word = 'J' Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'str' object does not support item assignment >>> word[2:] = 'py' Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: 'str' object does not support item assignment
If you need a different string, you should create a new one:
>>> 'J' + word[1:] 'Jython' >>> word[:2] + 'py' 'Pypy'
The built-in function
>>> s = 'supercalifragilisticexpialidocious' >>> len(s) 34
Python knows a number of compound data types, used to group together other values. The most versatile is the list, which can be written as a list of comma-separated values (items) between square brackets. Lists might contain items of different types, but usually the items all have the same type.
>>> squares = [1, 4, 9, 16, 25] >>> squares [1, 4, 9, 16, 25]
Like strings (and all other built-in sequence types), lists can be indexed and sliced:
>>> squares # indexing returns the item 1 >>> squares[-1] 25 >>> squares[-3:] # slicing returns a new list [9, 16, 25]
All slice operations return a new list containing the requested elements. This means that the following slice returns a shallow copy of the list:
>>> squares[:] [1, 4, 9, 16, 25]
Lists also support operations like concatenation:
>>> squares + [36, 49, 64, 81, 100] [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
Unlike strings, which are immutable, lists are a mutable type, i.e. it is possible to change their content:
>>> cubes = [1, 8, 27, 65, 125] # something's wrong here >>> 4 ** 3 # the cube of 4 is 64, not 65! 64 >>> cubes = 64 # replace the wrong value >>> cubes [1, 8, 27, 64, 125]
You can also add new items at the end of the list, by using the
>>> cubes.append(216) # add the cube of 6 >>> cubes.append(7 ** 3) # and the cube of 7 >>> cubes [1, 8, 27, 64, 125, 216, 343]
Assignment to slices is also possible, and this can even change the size of the list or clear it entirely:
>>> letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g'] >>> letters ['a', 'b', 'c', 'd', 'e', 'f', 'g'] >>> # replace some values >>> letters[2:5] = ['C', 'D', 'E'] >>> letters ['a', 'b', 'C', 'D', 'E', 'f', 'g'] >>> # now remove them >>> letters[2:5] =  >>> letters ['a', 'b', 'f', 'g'] >>> # clear the list by replacing all the elements with an empty list >>> letters[:] =  >>> letters 
The built-in function
>>> letters = ['a', 'b', 'c', 'd'] >>> len(letters) 4
It is possible to nest lists (create lists containing other lists), for example:
>>> a = ['a', 'b', 'c'] >>> n = [1, 2, 3] >>> x = [a, n] >>> x [['a', 'b', 'c'], [1, 2, 3]] >>> x ['a', 'b', 'c'] >>> x 'b'
3.2. First Steps Towards Programming¶
Of course, we can use Python for more complicated tasks than adding two and two together. For instance, we can write an initial sub-sequence of the Fibonacci series as follows:
>>> # Fibonacci series: ... # the sum of two elements defines the next ... a, b = 0, 1 >>> while a < 10: ... print(a) ... a, b = b, a+b ... 0 1 1 2 3 5 8
This example introduces several new features.
languages, special characters such as