Python: Pengantar Python Secara Informal
Dalam contoh berikut, input dan output dibedakan dengan ada atau tidak adanya prompt (>>> dan ...): untuk mengulang contoh, kita harus mengetikkan semuanya setelah prompt, ketika muncul prompt; baris yang tidak dimulai dengan prompt adalah output dari interpreter. Perhatikan bahwa prompt sekunder pada baris dengan sendirinya dalam contoh berarti kita harus mengetikkan baris kosong; ini digunakan untuk mengakhiri perintah multi-line.
Banyak contoh dalam manual ini, bahkan kita masuk pada prompt interaktif, termasuk komentar. Komentar di Python mulai dengan karakter hash, #, dan dapat di tarik ke akhir baris fisik. Sebuah komentar dapat saja muncul pada awal baris atau mengikuti spasi atau kode, tapi tidak dalam string literal. Sebuah karakter hash dalam string literal hanya karakter hash. Karena komentar yang untuk memperjelas kode dan tidak ditafsirkan oleh Python, mereka dapat dihilangkan saat mengetik di contoh.
Beberapa contoh:
# 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."
Python sebagai Kalkulator
Coba beberapa perintah python berikut. Start interpreter dan tunggu sampai dapat primary prompt >>>
Angka
Interpreter bertindak sebagai kalkulator sederhana: Kita bisa mengetik sebuah ekspresi dan interpreter akan menulis nilai. sintaks ekspresi sangat mudah: operator +, -, * dan / kerja seperti di kebanyakan bahasa lain (misalnya, Pascal atau C); kurung (()) dapat digunakan untuk mengelompokkan. Sebagai contoh:
>>> 2 + 2 4 >>> 50 - 5*6 20 >>> (50 - 5*6) / 4 5.0 >>> 8 / 5 # division always returns a floating point number 1.6
Angka bilangan bulat (misalnya 2, 4, 20) memiliki tipe int, angka dengan bagian pecahan (misalnya 5.0, 1.6) memiliki tipe float. Kita akan melihat lebih lanjut tentang jenis numerik nanti di tutorial.
Pembangian (/) selalu mengembalikan floating point. Untuk melakukan pembulatan pembagian dan mendapatkan hasil bilangan bulat (membuang apapun hasil pecahan) Anda dapat menggunakan operator //; untuk menghitung sisa Anda dapat menggunakan%:
>>> 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 # result * divisor + remainder 17
Dengan Python, adalah mungkin untuk menggunakan operator ** untuk menghitung pangkat:
>>> 5 ** 2 # 5 squared 25 >>> 2 ** 7 # 2 to the power of 7 128
Tanda sama dengan (=) digunakan untuk menetapkan nilai ke variabel. Setelah itu, tidak ada hasil ditampilkan sebelum prompt interaktif berikutnya:
>>> width = 20 >>> height = 5 * 9 >>> width * height 900
Jika variabel tidak di "defined" (assign nilai), mencoba untuk menggunakannya akan memberikan kesalahan:
>>> n # try to access an undefined variable Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'n' is not defined
Ada dukungan penuh untuk floating point; operator dengan jenis operand campuran untk mengkonversikan operand integer untuk floating point:
>>> 3 * 3.75 / 1.5 7.5 >>> 7.0 / 2 3.5
Dalam mode interaktif, ekspresi dicetak terakhir di assign untuk variabel _. Ini berarti bahwa ketika kita menggunakan Python sebagai kalkulator, itu agak lebih mudah untuk melanjutkan perhitungan, misalnya:
>>> tax = 12.5 / 100 >>> price = 100.50 >>> price * tax 12.5625 >>> price + _ 113.0625 >>> round(_, 2) 113.06
Variabel ini harus diperlakukan sebagai read-only oleh pengguna. Tidak secara eksplisit memberikan nilai pada variabel tersebut - kita akan membuat variabel lokal independen dengan nama yang sama masking built-in variabel dengan perilaku ajaibnya.
In addition to int and float, Python supports other types of numbers, such as Decimal and Fraction. Python also has built-in support for complex numbers, and uses the j or J suffix to indicate the imaginary part (e.g. 3+5j).
Strings
Besides numbers, Python can also manipulate strings, which can be expressed in several ways. They can be enclosed in single quotes ('...') or double quotes ("...") with the same result [2]. \ can be used to escape 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," he said.' '"Yes," he said.' >>> "\"Yes,\" he said." '"Yes," he said.' >>> '"Isn\'t," she said.' '"Isn\'t," she 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 print() function produces a more readable output, by omitting the enclosing quotes and by printing escaped and special characters: >>>
>>> '"Isn\'t," she said.'
'"Isn\'t," she said.'
>>> print('"Isn\'t," she said.')
"Isn't," she 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 \ to be interpreted as special characters, you can use raw strings by adding an r before the first quote: >>>
>>> 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: """...""" or .... End of lines are automatically included in the string, but it’s possible to prevent this by adding a \ at the end of the line. The following example:
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 + operator, and repeated with *: >>>
>>> # 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. >>>
>>> 'Py' 'thon' 'Python'
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
...
SyntaxError: invalid syntax
>>> ('un' * 3) 'ium'
...
SyntaxError: invalid syntax
If you want to concatenate variables or a variable and a literal, use +: >>>
>>> prefix + 'thon' 'Python'
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.'
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[0] # character in position 0 'P' >>> word[5] # 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'
Note how the start is always included, and the end always excluded. This makes sure that s[:i] + s[i:] is always equal to s: >>>
>>> word[:2] + word[2:] 'Python' >>> word[:4] + word[4:] 'Python'
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'
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 word[1:3] is 2.
Attempting to use an index that is too large will result in an error: >>>
>>> word[42] # 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[0] = 'J' ... TypeError: 'str' object does not support item assignment >>> word[2:] = 'py' ... 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 len() returns the length of a string: >>>
>>> s = 'supercalifragilisticexpialidocious' >>> len(s) 34
See also
Text Sequence Type — str
Strings are examples of sequence types, and support the common operations supported by such types.
String Methods
Strings support a large number of methods for basic transformations and searching.
Formatted string literals
String literals that have embedded expressions.
Format String Syntax
Information about string formatting with str.format().
printf-style String Formatting
The old formatting operations invoked when strings and Unicode strings are the left operand of the % operator are described in more detail here.
3.1.3. Lists
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 type), lists can be indexed and sliced: >>>
>>> squares[0] # 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 new (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[3] = 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 append() method (we will see more about methods later): >>>
>>> 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 len() also applies to lists: >>>
>>> 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[0] ['a', 'b', 'c'] >>> x[0][1] '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 b < 10: ... print(b) ... a, b = b, a+b ... 1 1 2 3 5 8
This example introduces several new features.
The first line contains a multiple assignment: the variables a and b simultaneously get the new values 0 and 1. On the last line this is used again, demonstrating that the expressions on the right-hand side are all evaluated first before any of the assignments take place. The right-hand side expressions are evaluated from the left to the right.
The while loop executes as long as the condition (here: b < 10) remains true. In Python, like in C, any non-zero integer value is true; zero is false. The condition may also be a string or list value, in fact any sequence; anything with a non-zero length is true, empty sequences are false. The test used in the example is a simple comparison. The standard comparison operators are written the same as in C: < (less than), > (greater than), == (equal to), <= (less than or equal to), >= (greater than or equal to) and != (not equal to).
The body of the loop is indented: indentation is Python’s way of grouping statements. At the interactive prompt, you have to type a tab or space(s) for each indented line. In practice you will prepare more complicated input for Python with a text editor; all decent text editors have an auto-indent facility. When a compound statement is entered interactively, it must be followed by a blank line to indicate completion (since the parser cannot guess when you have typed the last line). Note that each line within a basic block must be indented by the same amount.
The print() function writes the value of the argument(s) it is given. It differs from just writing the expression you want to write (as we did earlier in the calculator examples) in the way it handles multiple arguments, floating point quantities, and strings. Strings are printed without quotes, and a space is inserted between items, so you can format things nicely, like this: >>>
>>> i = 256*256
>>> print('The value of i is', i)
The value of i is 65536
The keyword argument end can be used to avoid the newline after the output, or end the output with a different string: >>>
>>> a, b = 0, 1
>>> while b < 1000:
... print(b, end=',')
... a, b = b, a+b
...
1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,
Footnotes [1] Since ** has higher precedence than -, -3**2 will be interpreted as -(3**2) and thus result in -9. To avoid this and get 9, you can use (-3)**2. [2] Unlike other languages, special characters such as \n have the same meaning with both single ('...') and double ("...") quotes. The only difference between the two is that within single quotes you don’t need to escape " (but you have to escape \') and vice versa.