7. Masukan dan Keluaran¶
Ada beberapa cara untuk mempresentasikan keluaran suatu program; data dapat dicetak dalam bentuk yang dapat dibaca manusia, atau ditulis ke berkas untuk digunakan di masa mendatang. Bab ini akan membahas beberapa kemungkinan.
7.1. Pemformatan Keluaran yang Lebih Menarik¶
So far we've encountered two ways of writing values: expression statements and
the print statement. (A third way is using the write() method
of file objects; the standard output file can be referenced as sys.stdout.
See the Library Reference for more information on this.)
Often you'll want more control over the formatting of your output than simply
printing space-separated values. There are two ways to format your output; the
first way is to do all the string handling yourself; using string slicing and
concatenation operations you can create any layout you can imagine. The
string types have some methods that perform useful operations for padding
strings to a given column width; these will be discussed shortly. The second
way is to use the str.format() method.
The string module contains a Template class which offers
yet another way to substitute values into strings.
One question remains, of course: how do you convert values to strings? Luckily,
Python has ways to convert any value to a string: pass it to the repr()
or str() functions.
The str() function is meant to return representations of values which are
fairly human-readable, while repr() is meant to generate representations
which can be read by the interpreter (or will force a SyntaxError if
there is no equivalent syntax). For objects which don't have a particular
representation for human consumption, str() will return the same value as
repr(). Many values, such as numbers or structures like lists and
dictionaries, have the same representation using either function. Strings and
floating point numbers, in particular, have two distinct representations.
Beberapa contoh:
>>> s = 'Hello, world.'
>>> str(s)
'Hello, world.'
>>> repr(s)
"'Hello, world.'"
>>> str(1.0/7.0)
'0.142857142857'
>>> repr(1.0/7.0)
'0.14285714285714285'
>>> x = 10 * 3.25
>>> y = 200 * 200
>>> s = 'The value of x is ' + repr(x) + ', and y is ' + repr(y) + '...'
>>> print s
The value of x is 32.5, and y is 40000...
>>> # The repr() of a string adds string quotes and backslashes:
... hello = 'hello, world\n'
>>> hellos = repr(hello)
>>> print hellos
'hello, world\n'
>>> # The argument to repr() may be any Python object:
... repr((x, y, ('spam', 'eggs')))
"(32.5, 40000, ('spam', 'eggs'))"
Here are two ways to write a table of squares and cubes:
>>> for x in range(1, 11):
... print repr(x).rjust(2), repr(x*x).rjust(3),
... # Note trailing comma on previous line
... print repr(x*x*x).rjust(4)
...
1 1 1
2 4 8
3 9 27
4 16 64
5 25 125
6 36 216
7 49 343
8 64 512
9 81 729
10 100 1000
>>> for x in range(1,11):
... print '{0:2d} {1:3d} {2:4d}'.format(x, x*x, x*x*x)
...
1 1 1
2 4 8
3 9 27
4 16 64
5 25 125
6 36 216
7 49 343
8 64 512
9 81 729
10 100 1000
(Note that in the first example, one space between each column was added by the
way print works: by default it adds spaces between its arguments.)
This example demonstrates the str.rjust() method of string
objects, which right-justifies a string in a field of a given width by padding
it with spaces on the left. There are similar methods str.ljust() and
str.center(). These methods do not write anything, they just return a
new string. If the input string is too long, they don't truncate it, but
return it unchanged; this will mess up your column lay-out but that's usually
better than the alternative, which would be lying about a value. (If you
really want truncation you can always add a slice operation, as in
x.ljust(n)[:n].)
Ada metode lain, str.zfill(), yang melapisi string numerik di sebelah kiri dengan nol. Itu mengerti tentang tanda plus dan minus:
>>> '12'.zfill(5)
'00012'
>>> '-3.14'.zfill(7)
'-003.14'
>>> '3.14159265359'.zfill(5)
'3.14159265359'
Penggunaan dasar metode str.format() terlihat seperti ini:
>>> print 'We are the {} who say "{}!"'.format('knights', 'Ni')
We are the knights who say "Ni!"
The brackets and characters within them (called format fields) are replaced with
the objects passed into the str.format() method. A number in the
brackets refers to the position of the object passed into the
str.format() method.
>>> print '{0} and {1}'.format('spam', 'eggs')
spam and eggs
>>> print '{1} and {0}'.format('spam', 'eggs')
eggs and spam
Jika argumen kata kunci keyword argument digunakan dalam metode str.format(), nilainya dirujuk dengan menggunakan nama argumen.
>>> print 'This {food} is {adjective}.'.format(
... food='spam', adjective='absolutely horrible')
This spam is absolutely horrible.
Argumen posisi dan kata kunci dapat dikombinasikan secara bergantian:
>>> print 'The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred',
... other='Georg')
The story of Bill, Manfred, and Georg.
'!s' (apply str()) and '!r' (apply repr()) can be used to
convert the value before it is formatted.
>>> import math
>>> print 'The value of PI is approximately {}.'.format(math.pi)
The value of PI is approximately 3.14159265359.
>>> print 'The value of PI is approximately {!r}.'.format(math.pi)
The value of PI is approximately 3.141592653589793.
An optional ':' and format specifier can follow the field name. This allows
greater control over how the value is formatted. The following example
rounds Pi to three places after the decimal.
>>> import math
>>> print 'The value of PI is approximately {0:.3f}.'.format(math.pi)
The value of PI is approximately 3.142.
Passing an integer after the ':' will cause that field to be a minimum
number of characters wide. This is useful for making tables pretty.
>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678}
>>> for name, phone in table.items():
... print '{0:10} ==> {1:10d}'.format(name, phone)
...
Jack ==> 4098
Dcab ==> 7678
Sjoerd ==> 4127
Jika Anda memiliki string format yang sangat panjang yang tidak ingin Anda pisahkan, alangkah baiknya jika Anda bisa mereferensikan variabel yang akan diformat berdasarkan nama alih-alih berdasarkan posisi. Ini dapat dilakukan hanya dengan melewatkan dict dan menggunakan tanda kurung siku '[]' untuk mengakses kunci dari dict
>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
>>> print ('Jack: {0[Jack]:d}; Sjoerd: {0[Sjoerd]:d}; '
... 'Dcab: {0[Dcab]:d}'.format(table))
Jack: 4098; Sjoerd: 4127; Dcab: 8637678
Ini juga bisa dilakukan dengan memberikan tabel sebagai argumen kata kunci keyword argument dengan notasi '**'.
>>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 8637678}
>>> print 'Jack: {Jack:d}; Sjoerd: {Sjoerd:d}; Dcab: {Dcab:d}'.format(**table)
Jack: 4098; Sjoerd: 4127; Dcab: 8637678
Ini sangat berguna dalam kombinasi dengan fungsi bawaan vars(), yang mengembalikan dictionary yang berisi semua variabel lokal.
Untuk ikhtisar lengkap pemformatan string dengan str.format(), lihat Format String Syntax.
7.1.1. Pemformatan string lama¶
Operator % juga dapat digunakan untuk pemformatan string. Ini menafsirkan argumen kiri seperti gaya sprintf()format string untuk diterapkan pada argumen yang benar, dan mengembalikan string yang dihasilkan dari operasi pemformatan ini. Sebagai contoh:
>>> import math
>>> print 'The value of PI is approximately %5.3f.' % math.pi
The value of PI is approximately 3.142.
More information can be found in the String Formatting Operations section.
7.2. Membaca dan Menulis Berkas¶
open() returns a file object, and is most commonly used with two
arguments: open(filename, mode).
>>> f = open('workfile', 'w')
>>> print f
<open file 'workfile', mode 'w' at 80a0960>
Argumen pertama adalah string yang berisi nama file. Argumen kedua adalah string lain yang berisi beberapa karakter yang menggambarkan cara berkas akan digunakan. mode dapat 'r' ketika file hanya akan dibaca, 'w' untuk hanya menulis (berkas yang ada dengan nama yang sama akan dihapus), dan 'a' membuka berkas untuk ditambahkan; setiap data yang ditulis ke file secara otomatis ditambahkan ke bagian akhir. 'r+' membuka berkas untuk membaca dan menulis. Argumen mode adalah opsional; 'r' akan diasumsikan jika dihilangkan.
On Windows, 'b' appended to the mode opens the file in binary mode, so there
are also modes like 'rb', 'wb', and 'r+b'. Python on Windows makes
a distinction between text and binary files; the end-of-line characters in text
files are automatically altered slightly when data is read or written. This
behind-the-scenes modification to file data is fine for ASCII text files, but
it'll corrupt binary data like that in JPEG or EXE files. Be
very careful to use binary mode when reading and writing such files. On Unix,
it doesn't hurt to append a 'b' to the mode, so you can use it
platform-independently for all binary files.
7.2.1. Metode Objek Berkas¶
Sisa contoh di bagian ini akan menganggap bahwa objek berkas bernama f telah dibuat.
To read a file's contents, call f.read(size), which reads some quantity of
data and returns it as a string. size is an optional numeric argument. When
size is omitted or negative, the entire contents of the file will be read and
returned; it's your problem if the file is twice as large as your machine's
memory. Otherwise, at most size bytes are read and returned. If the end of
the file has been reached, f.read() will return an empty string ("").
>>> f.read()
'This is the entire file.\n'
>>> f.read()
''
f.readline() reads a single line from the file; a newline character (\n)
is left at the end of the string, and is only omitted on the last line of the
file if the file doesn't end in a newline. This makes the return value
unambiguous; if f.readline() returns an empty string, the end of the file
has been reached, while a blank line is represented by '\n', a string
containing only a single newline.
>>> f.readline()
'This is the first line of the file.\n'
>>> f.readline()
'Second line of the file\n'
>>> f.readline()
''
Untuk membaca baris dari file, Anda dapat mengulangi objek berkas. Ini hemat memori, cepat, dan mengarah ke kode sederhana
>>> for line in f:
print line,
This is the first line of the file.
Second line of the file
Jika Anda ingin membaca semua baris file dalam daftar list, Anda juga dapat menggunakan list(f) atau f.readlines().
f.write(string) writes the contents of string to the file, returning
None.
>>> f.write('This is a test\n')
To write something other than a string, it needs to be converted to a string first:
>>> value = ('the answer', 42)
>>> s = str(value)
>>> f.write(s)
f.tell() returns an integer giving the file object's current position in the
file, measured in bytes from the beginning of the file. To change the file
object's position, use f.seek(offset, from_what). The position is computed
from adding offset to a reference point; the reference point is selected by
the from_what argument. A from_what value of 0 measures from the beginning
of the file, 1 uses the current file position, and 2 uses the end of the file as
the reference point. from_what can be omitted and defaults to 0, using the
beginning of the file as the reference point.
>>> f = open('workfile', 'r+')
>>> f.write('0123456789abcdef')
>>> f.seek(5) # Go to the 6th byte in the file
>>> f.read(1)
'5'
>>> f.seek(-3, 2) # Go to the 3rd byte before the end
>>> f.read(1)
'd'
When you're done with a file, call f.close() to close it and free up any
system resources taken up by the open file. After calling f.close(),
attempts to use the file object will automatically fail.
>>> f.close()
>>> f.read()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: I/O operation on closed file
It is good practice to use the with keyword when dealing with file
objects. This has the advantage that the file is properly closed after its
suite finishes, even if an exception is raised on the way. It is also much
shorter than writing equivalent try-finally blocks:
>>> with open('workfile', 'r') as f:
... read_data = f.read()
>>> f.closed
True
Objek file memiliki beberapa metode tambahan, seperti isatty() dan truncate() yang lebih jarang digunakan; bacalah Referensi Pustaka untuk panduan lengkap untuk objek berkas.
7.2.2. Menyimpan data terstruktur dengan json¶
String dapat dengan mudah ditulis dan dibaca dari file. Angka membutuhkan sedikit usaha, karena metode read() hanya mengembalikan string, yang harus diteruskan ke fungsi seperti int(), yang mengambil string seperti '123' dan mengembalikan nilai numerik 123. Ketika Anda ingin menyimpan tipe data yang lebih kompleks seperti daftar list dan dictionary bersarang, penguraian dan pembuatan serialisasi dengan tangan menjadi rumit.
Alih-alih membuat pengguna terus-menerus menulis dan men-debug kode untuk menyimpan tipe data yang rumit ke berkas, Python memungkinkan Anda untuk menggunakan format pertukaran data populer yang disebut JSON (JavaScript Object Notation). Modul standar bernama json dapat mengambil hierarki data Python, dan mengubahnya menjadi representasi string; proses ini disebut serializing. Merekonstruksi data dari representasi string disebut deserializing. Antara serializing dan deserializing, string yang mewakili objek mungkin telah disimpan dalam berkas atau data, atau dikirim melalui koneksi jaringan ke beberapa mesin yang jauh.
Catatan
Format JSON umumnya digunakan oleh aplikasi modern untuk memungkinkan pertukaran data. Banyak programmer sudah terbiasa dengannya, yang membuatnya menjadi pilihan yang baik untuk interoperabilitas.
Jika Anda memiliki objek x, Anda dapat melihat representasi string JSON dengan baris kode sederhana:
>>> import json
>>> json.dumps([1, 'simple', 'list'])
'[1, "simple", "list"]'
Another variant of the dumps() function, called dump(),
simply serializes the object to a file. So if f is a file object
opened for writing, we can do this:
json.dump(x, f)
To decode the object again, if f is a file object which has
been opened for reading:
x = json.load(f)
Teknik serialisasi sederhana ini dapat menangani daftar list dan dictionary, tetapi membuat serialisasi instance kelas yang berubah-ubah arbitrary di JSON membutuhkan sedikit usaha ekstra. Referensi untuk modul json berisi penjelasan tentang ini.
Lihat juga
Pickle - modul pickle
Berlawanan dengan JSON, pickle adalah protokol yang memungkinkan serialisasi objek Python yang semena-mena arbitrarily kompleks. Dengan demikian, ini khusus untuk Python dan tidak dapat digunakan untuk berkomunikasi dengan aplikasi yang ditulis dalam bahasa lain. Ini juga tidak aman secara bawaan: deserializing pickle data yang berasal dari sumber yang tidak dipercaya dapat mengeksekusi kode semena-mena arbitrary, jika data dibuat oleh penyerang yang terampil.