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diff --git a/Doc/tutorial/introduction.rst b/Doc/tutorial/introduction.rst index 2a16661..2ff776e 100644 --- a/Doc/tutorial/introduction.rst +++ b/Doc/tutorial/introduction.rst @@ -11,8 +11,6 @@ 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. -.. index:: single: # (hash); comment - Many of the examples in this manual, even those entered at the interactive prompt, include comments. Comments in Python start with the hash character, ``#``, and extend to the end of the physical line. A comment may appear at the @@ -53,24 +51,28 @@ For example:: 4 >>> 50 - 5*6 20 - >>> (50 - 5*6) / 4 + >>> (50 - 5.0*6) / 4 5.0 - >>> 8 / 5 # division always returns a floating point number + >>> 8 / 5.0 1.6 The integer numbers (e.g. ``2``, ``4``, ``20``) have type :class:`int`, the ones with a fractional part (e.g. ``5.0``, ``1.6``) have type :class:`float`. We will see more about numeric types later in the tutorial. -Division (``/``) always returns a float. To do :term:`floor division` and -get an integer result (discarding any fractional result) you can use the ``//`` -operator; to calculate the remainder you can use ``%``:: +The return type of a division (``/``) operation depends on its operands. If +both operands are of type :class:`int`, :term:`floor division` is performed +and an :class:`int` is returned. If either operand is a :class:`float`, +classic division is performed and a :class:`float` is returned. The ``//`` +operator is also provided for doing floor division no matter what the +operands are. The remainder can be calculated with the ``%`` operator:: - >>> 17 / 3 # classic division returns a float - 5.666666666666667 - >>> - >>> 17 // 3 # floor division discards the fractional part + >>> 17 / 3 # int / int -> int 5 + >>> 17 / 3.0 # int / float -> float + 5.666666666666667 + >>> 17 // 3.0 # explicit floor division discards the fractional part + 5.0 >>> 17 % 3 # the % operator returns the remainder of the division 2 >>> 5 * 3 + 2 # result * divisor + remainder @@ -102,8 +104,10 @@ give you an error:: There is full support for floating point; operators with mixed type operands convert the integer operand to floating point:: - >>> 4 * 3.75 - 1 - 14.0 + >>> 3 * 3.75 / 1.5 + 7.5 + >>> 7.0 / 2 + 3.5 In interactive mode, the last printed expression is assigned to the variable ``_``. This means that when you are using Python as a desk calculator, it is @@ -157,18 +161,18 @@ 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 :func:`print` function produces a more +enclosed in single quotes. The :keyword:`print` statement produces a more readable output, by omitting the enclosing quotes and by printing escaped and special characters:: >>> '"Isn\'t," they said.' '"Isn\'t," they said.' - >>> print('"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 + >>> s # without print, \n is included in the output 'First line.\nSecond line.' - >>> print(s) # with print(), \n produces a new line + >>> print s # with print, \n produces a new line First line. Second line. @@ -176,10 +180,10 @@ 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! + >>> print 'C:\some\name' # here \n means newline! C:\some ame - >>> print(r'C:\some\name') # note the r before the quote + >>> 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: @@ -187,11 +191,11 @@ String literals can span multiple lines. One way is using triple-quotes: included in the string, but it's possible to prevent this by adding a ``\`` at the end of the line. The following example:: - print("""\ + 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): @@ -225,14 +229,10 @@ 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 ``+``:: @@ -262,7 +262,7 @@ Indices may also be negative numbers, to start counting from the right:: 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:: +to obtain individual characters, *slicing* allows you to obtain a substring:: >>> word[0:2] # characters from position 0 (included) to 2 (excluded) 'Py' @@ -326,12 +326,10 @@ Python strings cannot be changed --- they are :term:`immutable`. Therefore, assigning to an indexed position in the string results in an error:: >>> word[0] = '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:: @@ -350,25 +348,121 @@ The built-in function :func:`len` returns the length of a string:: .. seealso:: - :ref:`textseq` - Strings are examples of *sequence types*, and support the common - operations supported by such types. + :ref:`typesseq` + Strings, and the Unicode strings described in the next section, are + examples of *sequence types*, and support the common operations supported + by such types. :ref:`string-methods` - Strings support a large number of methods for + Both strings and Unicode strings support a large number of methods for basic transformations and searching. - :ref:`f-strings` - String literals that have embedded expressions. - :ref:`formatstrings` Information about string formatting with :meth:`str.format`. - :ref:`old-string-formatting` - The old formatting operations invoked when strings are + :ref:`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. +.. _tut-unicodestrings: + +Unicode Strings +--------------- + +.. sectionauthor:: Marc-Andre Lemburg <mal@lemburg.com> + + +Starting with Python 2.0 a new data type for storing text data is available to +the programmer: the Unicode object. It can be used to store and manipulate +Unicode data (see http://www.unicode.org/) and integrates well with the existing +string objects, providing auto-conversions where necessary. + +Unicode has the advantage of providing one ordinal for every character in every +script used in modern and ancient texts. Previously, there were only 256 +possible ordinals for script characters. Texts were typically bound to a code +page which mapped the ordinals to script characters. This lead to very much +confusion especially with respect to internationalization (usually written as +``i18n`` --- ``'i'`` + 18 characters + ``'n'``) of software. Unicode solves +these problems by defining one code page for all scripts. + +Creating Unicode strings in Python is just as simple as creating normal +strings:: + + >>> u'Hello World !' + u'Hello World !' + +The small ``'u'`` in front of the quote indicates that a Unicode string is +supposed to be created. If you want to include special characters in the string, +you can do so by using the Python *Unicode-Escape* encoding. The following +example shows how:: + + >>> u'Hello\u0020World !' + u'Hello World !' + +The escape sequence ``\u0020`` indicates to insert the Unicode character with +the ordinal value 0x0020 (the space character) at the given position. + +Other characters are interpreted by using their respective ordinal values +directly as Unicode ordinals. If you have literal strings in the standard +Latin-1 encoding that is used in many Western countries, you will find it +convenient that the lower 256 characters of Unicode are the same as the 256 +characters of Latin-1. + +For experts, there is also a raw mode just like the one for normal strings. You +have to prefix the opening quote with 'ur' to have Python use the +*Raw-Unicode-Escape* encoding. It will only apply the above ``\uXXXX`` +conversion if there is an uneven number of backslashes in front of the small +'u'. :: + + >>> ur'Hello\u0020World !' + u'Hello World !' + >>> ur'Hello\\u0020World !' + u'Hello\\\\u0020World !' + +The raw mode is most useful when you have to enter lots of backslashes, as can +be necessary in regular expressions. + +Apart from these standard encodings, Python provides a whole set of other ways +of creating Unicode strings on the basis of a known encoding. + +.. index:: builtin: unicode + +The built-in function :func:`unicode` provides access to all registered Unicode +codecs (COders and DECoders). Some of the more well known encodings which these +codecs can convert are *Latin-1*, *ASCII*, *UTF-8*, and *UTF-16*. The latter two +are variable-length encodings that store each Unicode character in one or more +bytes. The default encoding is normally set to ASCII, which passes through +characters in the range 0 to 127 and rejects any other characters with an error. +When a Unicode string is printed, written to a file, or converted with +:func:`str`, conversion takes place using this default encoding. :: + + >>> u"abc" + u'abc' + >>> str(u"abc") + 'abc' + >>> u"äöü" + u'\xe4\xf6\xfc' + >>> str(u"äöü") + Traceback (most recent call last): + File "<stdin>", line 1, in ? + UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-2: ordinal not in range(128) + +To convert a Unicode string into an 8-bit string using a specific encoding, +Unicode objects provide an :func:`encode` method that takes one argument, the +name of the encoding. Lowercase names for encodings are preferred. :: + + >>> u"äöü".encode('utf-8') + '\xc3\xa4\xc3\xb6\xc3\xbc' + +If you have data in a specific encoding and want to produce a corresponding +Unicode string from it, you can use the :func:`unicode` function with the +encoding name as the second argument. :: + + >>> unicode('\xc3\xa4\xc3\xb6\xc3\xbc', 'utf-8') + u'\xe4\xf6\xfc' + + .. _tut-lists: Lists @@ -383,7 +477,7 @@ items of different types, but usually the items all have the same type. :: >>> squares [1, 4, 9, 16, 25] -Like strings (and all other built-in :term:`sequence` types), lists can be +Like strings (and all other built-in :term:`sequence` type), lists can be indexed and sliced:: >>> squares[0] # indexing returns the item @@ -394,13 +488,12 @@ indexed and sliced:: [9, 16, 25] All slice operations return a new list containing the requested elements. This -means that the following slice returns a -:ref:`shallow copy <shallow_vs_deep_copy>` of the list:: +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:: +Lists also supports operations like concatenation:: >>> squares + [36, 49, 64, 81, 100] [1, 4, 9, 16, 25, 36, 49, 64, 81, 100] @@ -467,18 +560,16 @@ 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 <https://en.wikipedia.org/wiki/Fibonacci_number>`_ -as follows:: +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) + >>> while b < 10: + ... print b ... a, b = b, a+b ... - 0 1 1 2 @@ -494,7 +585,7 @@ This example introduces several new features. first before any of the assignments take place. The right-hand side expressions are evaluated from the left to the right. -* The :keyword:`while` loop executes as long as the condition (here: ``a < 10``) +* The :keyword:`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 @@ -512,27 +603,27 @@ This example introduces several new features. guess when you have typed the last line). Note that each line within a basic block must be indented by the same amount. -* The :func:`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:: +* The :keyword:`print` statement writes the value of the expression(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 expressions + 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) + >>> 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 trailing comma avoids the newline after the output:: >>> a, b = 0, 1 - >>> while a < 1000: - ... print(a, end=',') + >>> while b < 1000: + ... print b, ... a, b = b, a+b ... - 0,1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987, + 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 + Note that the interpreter inserts a newline before it prints the next prompt if + the last line was not completed. .. rubric:: Footnotes |