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Python 3.6.5 Documentation > Input and Output

Input and Output
****************

There are several ways to present the output of a program; data can be
printed in a human-readable form, or written to a file for future use.
This chapter will discuss some of the possibilities.

Fancier Output Formatting
=========================

So far we’ve encountered two ways of writing values: *expression
statements* and the "print()" function. (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 type has 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 formatted string
literals, or 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, in particular, have
two distinct representations.

Some examples:

>>> s = 'Hello, world.'
>>> str(s)
'Hello, world.'
>>> repr(s)
"'Hello, world.'"
>>> str(1/7)
'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), end=' ')
... # Note use of 'end' 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]".)

There is another method, "str.zfill()", which pads a numeric string on
the left with zeros. It understands about plus and minus signs:

>>> '12'.zfill(5)
'00012'
>>> '-3.14'.zfill(7)
'-003.14'
>>> '3.14159265359'.zfill(5)
'3.14159265359'

Basic usage of the "str.format()" method looks like this:

>>> 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 can be used to refer 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

If keyword arguments are used in the "str.format()" method, their
values are referred to by using the name of the argument.

>>> print('This {food} is {adjective}.'.format(
... food='spam', adjective='absolutely horrible'))
This spam is absolutely horrible.

Positional and keyword arguments can be arbitrarily combined:

>>> print('The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred',
other='Georg'))
The story of Bill, Manfred, and Georg.

"'!a'" (apply "ascii()"), "'!s'" (apply "str()") and "'!r'" (apply
"repr()") can be used to convert the value before it is formatted:

>>> contents = 'eels'
>>> print('My hovercraft is full of {}.'.format(contents))
My hovercraft is full of eels.
>>> print('My hovercraft is full of {!r}.'.format(contents))
My hovercraft is full of 'eels'.

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

If you have a really long format string that you don’t want to split
up, it would be nice if you could reference the variables to be
formatted by name instead of by position. This can be done by simply
passing the dict and using square brackets "'[]'" to access the keys

>>> 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

This could also be done by passing the table as keyword arguments with
the ‘**’ notation.

>>> 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

This is particularly useful in combination with the built-in function
"vars()", which returns a dictionary containing all local variables.

For a complete overview of string formatting with "str.format()", see
Format String Syntax.

Old string formatting
---------------------

The "%" operator can also be used for string formatting. It interprets
the left argument much like a "sprintf()"-style format string to be
applied to the right argument, and returns the string resulting from
this formatting operation. For example:

>>> 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 printf-style String Formatting
section.

Reading and Writing Files
=========================

"open()" returns a *file object*, and is most commonly used with two
arguments: "open(filename, mode)".

>>> f = open('workfile', 'w')

The first argument is a string containing the filename. The second
argument is another string containing a few characters describing the
way in which the file will be used. *mode* can be "'r'" when the file
will only be read, "'w'" for only writing (an existing file with the
same name will be erased), and "'a'" opens the file for appending; any
data written to the file is automatically added to the end. "'r+'"
opens the file for both reading and writing. The *mode* argument is
optional; "'r'" will be assumed if it’s omitted.

Normally, files are opened in *text mode*, that means, you read and
write strings from and to the file, which are encoded in a specific
encoding. If encoding is not specified, the default is platform
dependent (see "open()"). "'b'" appended to the mode opens the file in
*binary mode*: now the data is read and written in the form of bytes
objects. This mode should be used for all files that don’t contain
text.

In text mode, the default when reading is to convert platform-specific
line endings ("\n" on Unix, "\r\n" on Windows) to just "\n". When
writing in text mode, the default is to convert occurrences of "\n"
back to platform-specific line endings. This behind-the-scenes
modification to file data is fine for text files, but will corrupt
binary data like that in "JPEG" or "EXE" files. Be very careful to
use binary mode when reading and writing such files.

It is good practice to use the "with" keyword when dealing with file
objects. The advantage is that the file is properly closed after its
suite finishes, even if an exception is raised at some point. Using
"with" is also much shorter than writing equivalent "try"-"finally"
blocks:

>>> with open('workfile') as f:
... read_data = f.read()
>>> f.closed
True

If you’re not using the "with" keyword, then you should call
"f.close()" to close the file and immediately free up any system
resources used by it. If you don’t explicitly close a file, Python’s
garbage collector will eventually destroy the object and close the
open file for you, but the file may stay open for a while. Another
risk is that different Python implementations will do this clean-up at
different times.

After a file object is closed, either by a "with" statement or by
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

Methods of File Objects
-----------------------

The rest of the examples in this section will assume that a file
object called "f" has already been created.

To read a file’s contents, call "f.read(size)", which reads some
quantity of data and returns it as a string (in text mode) or bytes
object (in binary mode). *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()
''

For reading lines from a file, you can loop over the file object. This
is memory efficient, fast, and leads to simple code:

>>> for line in f:
... print(line, end='')
...
This is the first line of the file.
Second line of the file

If you want to read all the lines of a file in a list you can also use
"list(f)" or "f.readlines()".

"f.write(string)" writes the contents of *string* to the file,
returning the number of characters written.

>>> f.write('This is a test\n')
15

Other types of objects need to be converted – either to a string (in
text mode) or a bytes object (in binary mode) – before writing them:

>>> value = ('the answer', 42)
>>> s = str(value) # convert the tuple to string
>>> f.write(s)
18

"f.tell()" returns an integer giving the file object’s current
position in the file represented as number of bytes from the beginning
of the file when in binary mode and an opaque number when in text
mode.

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', 'rb+')
>>> f.write(b'0123456789abcdef')
16
>>> f.seek(5) # Go to the 6th byte in the file
5
>>> f.read(1)
b'5'
>>> f.seek(-3, 2) # Go to the 3rd byte before the end
13
>>> f.read(1)
b'd'

In text files (those opened without a "b" in the mode string), only
seeks relative to the beginning of the file are allowed (the exception
being seeking to the very file end with "seek(0, 2)") and the only
valid *offset* values are those returned from the "f.tell()", or zero.
Any other *offset* value produces undefined behaviour.

File objects have some additional methods, such as "isatty()" and
"truncate()" which are less frequently used; consult the Library
Reference for a complete guide to file objects.

Saving structured data with "json"
----------------------------------

Strings can easily be written to and read from a file. Numbers take a
bit more effort, since the "read()" method only returns strings, which
will have to be passed to a function like "int()", which takes a
string like "'123'" and returns its numeric value 123. When you want
to save more complex data types like nested lists and dictionaries,
parsing and serializing by hand becomes complicated.

Rather than having users constantly writing and debugging code to save
complicated data types to files, Python allows you to use the popular
data interchange format called JSON (JavaScript Object Notation). The
standard module called "json" can take Python data hierarchies, and
convert them to string representations; this process is called
*serializing*. Reconstructing the data from the string representation
is called *deserializing*. Between serializing and deserializing, the
string representing the object may have been stored in a file or data,
or sent over a network connection to some distant machine.

Note: The JSON format is commonly used by modern applications to
allow for data exchange. Many programmers are already familiar with
it, which makes it a good choice for interoperability.

If you have an object "x", you can view its JSON string representation
with a simple line of code:

>>> import json
>>> json.dumps([1, 'simple', 'list'])
'[1, "simple", "list"]'

Another variant of the "dumps()" function, called "dump()", simply
serializes the object to a *text file*. So if "f" is a *text file*
object opened for writing, we can do this:

json.dump(x, f)

To decode the object again, if "f" is a *text file* object which has
been opened for reading:

x = json.load(f)

This simple serialization technique can handle lists and dictionaries,
but serializing arbitrary class instances in JSON requires a bit of
extra effort. The reference for the "json" module contains an
explanation of this.