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diff --git a/Doc/tutorial/inputoutput.rst b/Doc/tutorial/inputoutput.rst index a404f4b..78d6ba3 100644 --- a/Doc/tutorial/inputoutput.rst +++ b/Doc/tutorial/inputoutput.rst @@ -15,46 +15,24 @@ Fancier Output Formatting ========================= So far we've encountered two ways of writing values: *expression statements* and -the :func:`print` function. (A third way is using the :meth:`write` method +the :keyword:`print` statement. (A third way is using the :meth:`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 several ways to format output. +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 :meth:`str.format` method. -* To use :ref:`formatted string literals <tut-f-strings>`, begin a string - with ``f`` or ``F`` before the opening quotation mark or triple quotation mark. - Inside this string, you can write a Python expression between ``{`` and ``}`` - characters that can refer to variables or literal values. +The :mod:`string` module contains a :class:`~string.Template` class which offers +yet another way to substitute values into strings. - :: - - >>> year = 2016 - >>> event = 'Referendum' - >>> f'Results of the {year} {event}' - 'Results of the 2016 Referendum' - -* The :meth:`str.format` method of strings requires more manual - effort. You'll still use ``{`` and ``}`` to mark where a variable - will be substituted and can provide detailed formatting directives, - but you'll also need to provide the information to be formatted. - - :: - - >>> yes_votes = 42_572_654 - >>> no_votes = 43_132_495 - >>> percentage = yes_votes / (yes_votes + no_votes) - >>> '{:-9} YES votes {:2.2%}'.format(yes_votes, percentage) - ' 42572654 YES votes 49.67%' - -* Finally, you can do all the string handling yourself by using string slicing and - concatenation operations to create any layout you can imagine. The - string type has some methods that perform useful operations for padding - strings to a given column width. - -When you don't need fancy output but just want a quick display of some -variables for debugging purposes, you can convert any value to a string with -the :func:`repr` or :func:`str` functions. +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 :func:`repr` +or :func:`str` functions. The :func:`str` function is meant to return representations of values which are fairly human-readable, while :func:`repr` is meant to generate representations @@ -62,8 +40,8 @@ which can be read by the interpreter (or will force a :exc:`SyntaxError` if there is no equivalent syntax). For objects which don't have a particular representation for human consumption, :func:`str` will return the same value as :func:`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. +dictionaries, have the same representation using either function. Strings and +floating point numbers, in particular, have two distinct representations. Some examples:: @@ -72,188 +50,159 @@ Some examples:: 'Hello, world.' >>> repr(s) "'Hello, world.'" - >>> str(1/7) + >>> 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) + >>> 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) + >>> print hellos 'hello, world\n' >>> # The argument to repr() may be any Python object: ... repr((x, y, ('spam', 'eggs'))) "(32.5, 40000, ('spam', 'eggs'))" -The :mod:`string` module contains a :class:`~string.Template` class that offers -yet another way to substitute values into strings, using placeholders like -``$x`` and replacing them with values from a dictionary, but offers much less -control of the formatting. - - -.. _tut-f-strings: - -Formatted String Literals -------------------------- +Here are two ways to write a table of squares and cubes:: -:ref:`Formatted string literals <f-strings>` (also called f-strings for -short) let you include the value of Python expressions inside a string by -prefixing the string with ``f`` or ``F`` and writing expressions as -``{expression}``. - -An optional format specifier can follow the expression. This allows greater -control over how the value is formatted. The following example rounds pi to -three places after the decimal:: - - >>> import math - >>> print(f'The value of pi is approximately {math.pi:.3f}.') - 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 columns line up. :: - - >>> table = {'Sjoerd': 4127, 'Jack': 4098, 'Dcab': 7678} - >>> for name, phone in table.items(): - ... print(f'{name:10} ==> {phone:10d}') + >>> 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) ... - Sjoerd ==> 4127 - Jack ==> 4098 - Dcab ==> 7678 + 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 -Other modifiers can be used to convert the value before it is formatted. -``'!a'`` applies :func:`ascii`, ``'!s'`` applies :func:`str`, and ``'!r'`` -applies :func:`repr`:: + >>> 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 - >>> animals = 'eels' - >>> print(f'My hovercraft is full of {animals}.') - My hovercraft is full of eels. - >>> print(f'My hovercraft is full of {animals!r}.') - My hovercraft is full of 'eels'. +(Note that in the first example, one space between each column was added by the +way :keyword:`print` works: by default it adds spaces between its arguments.) -For a reference on these format specifications, see -the reference guide for the :ref:`formatspec`. +This example demonstrates the :meth:`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 :meth:`str.ljust` and +:meth:`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]``.) -.. _tut-string-format: +There is another method, :meth:`str.zfill`, which pads a numeric string on the +left with zeros. It understands about plus and minus signs:: -The String format() Method --------------------------- + >>> '12'.zfill(5) + '00012' + >>> '-3.14'.zfill(7) + '-003.14' + >>> '3.14159265359'.zfill(5) + '3.14159265359' Basic usage of the :meth:`str.format` method looks like this:: - >>> print('We are the {} who say "{}!"'.format('knights', 'Ni')) + >>> 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 :meth:`str.format` method. A number in the -brackets can be used to refer to the position of the object passed into the +brackets refers to the position of the object passed into the :meth:`str.format` method. :: - >>> print('{0} and {1}'.format('spam', 'eggs')) + >>> print '{0} and {1}'.format('spam', 'eggs') spam and eggs - >>> print('{1} and {0}'.format('spam', 'eggs')) + >>> print '{1} and {0}'.format('spam', 'eggs') eggs and spam If keyword arguments are used in the :meth:`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')) + >>> 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')) + >>> print 'The story of {0}, {1}, and {other}.'.format('Bill', 'Manfred', + ... other='Georg') The story of Bill, Manfred, and Georg. +``'!s'`` (apply :func:`str`) and ``'!r'`` (apply :func:`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 + 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)) + >>> 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)) + >>> 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 :func:`vars`, which returns a dictionary containing all local variables. -As an example, the following lines produce a tidily-aligned -set of columns giving integers and their squares and cubes:: - - >>> 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 - For a complete overview of string formatting with :meth:`str.format`, see :ref:`formatstrings`. -Manual String Formatting ------------------------- - -Here's the same table of squares and cubes, formatted manually:: - - >>> 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 - -(Note that the one space between each column was added by the -way :func:`print` works: it always adds spaces between its arguments.) - -The :meth:`str.rjust` method of string objects right-justifies a string in a -field of a given width by padding it with spaces on the left. There are -similar methods :meth:`str.ljust` and :meth:`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, :meth:`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' - - Old string formatting --------------------- @@ -263,10 +212,10 @@ 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. + >>> 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 :ref:`old-string-formatting` section. +More information can be found in the :ref:`string-formatting` section. .. _tut-files: @@ -278,16 +227,13 @@ Reading and Writing Files builtin: open object: file -:func:`open` returns a :term:`file object`, and is most commonly used with -two arguments: ``open(filename, mode)``. +:func:`open` returns a file object, and is most commonly used with two +arguments: ``open(filename, mode)``. :: >>> f = open('workfile', 'w') - -.. XXX str(f) is <io.TextIOWrapper object at 0x82e8dc4> - - >>> print(f) + >>> print f <open file 'workfile', mode 'w' at 80a0960> The first argument is a string containing the filename. The second argument is @@ -299,51 +245,15 @@ 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 :dfn:`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 -:func:`open`). ``'b'`` appended to the mode opens the file in -:dfn:`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 -:file:`JPEG` or :file:`EXE` files. Be very careful to use binary mode when -reading and writing such files. - -It is good practice to use the :keyword:`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 :keyword:`!with` is also much shorter than writing -equivalent :keyword:`try`\ -\ :keyword:`finally` blocks:: - - >>> with open('workfile') as f: - ... read_data = f.read() - - >>> # We can check that the file has been automatically closed. - >>> f.closed - True - -If you're not using the :keyword:`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 :keyword:`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. +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 :file:`JPEG` or :file:`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. .. _tut-filemethods: @@ -355,13 +265,12 @@ 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* -characters (in text mode) or *size* bytes (in binary mode) are read and returned. -If the end of the file has been reached, ``f.read()`` will return an empty -string (``''``). :: +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' @@ -373,7 +282,7 @@ 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. :: +containing only a single newline. :: >>> f.readline() 'This is the first line of the file.\n' @@ -386,8 +295,8 @@ 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='') - ... + print line, + This is the first line of the file. Second line of the file @@ -395,47 +304,54 @@ 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. :: +``None``. :: >>> 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:: +To write something other than a string, it needs to be converted to a string +first:: >>> value = ('the answer', 42) - >>> s = str(value) # convert the tuple to string + >>> s = str(value) >>> 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, whence)``. The position is computed +``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 *whence* argument. A *whence* value of 0 measures from the beginning +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. *whence* can be omitted and defaults to 0, using the +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 = open('workfile', 'r+') + >>> f.write('0123456789abcdef') >>> f.seek(5) # Go to the 6th byte in the file - 5 >>> f.read(1) - b'5' + '5' >>> f.seek(-3, 2) # Go to the 3rd byte before the end - 13 >>> f.read(1) - b'd' + '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 :keyword:`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 :keyword:`try`\ -\ :keyword:`finally` blocks:: -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. + >>> with open('workfile', 'r') as f: + ... read_data = f.read() + >>> f.closed + True File objects have some additional methods, such as :meth:`~file.isatty` and :meth:`~file.truncate` which are less frequently used; consult the Library @@ -479,12 +395,12 @@ simple line of code:: '[1, "simple", "list"]' Another variant of the :func:`~json.dumps` function, called :func:`~json.dump`, -simply serializes the object to a :term:`text file`. So if ``f`` is a -:term:`text file` object opened for writing, we can do this:: +simply serializes the object to a file. So if ``f`` is a :term:`file object` +opened for writing, we can do this:: json.dump(x, f) -To decode the object again, if ``f`` is a :term:`text file` object which has +To decode the object again, if ``f`` is a :term:`file object` which has been opened for reading:: x = json.load(f) @@ -503,3 +419,4 @@ The reference for the :mod:`json` module contains an explanation of this. written in other languages. It is also insecure by default: deserializing pickle data coming from an untrusted source can execute arbitrary code, if the data was crafted by a skilled attacker. + |