1 2.. _introduction: 3 4************ 5Introduction 6************ 7 8This reference manual describes the Python programming language. It is not 9intended as a tutorial. 10 11While I am trying to be as precise as possible, I chose to use English rather 12than formal specifications for everything except syntax and lexical analysis. 13This should make the document more understandable to the average reader, but 14will leave room for ambiguities. Consequently, if you were coming from Mars and 15tried to re-implement Python from this document alone, you might have to guess 16things and in fact you would probably end up implementing quite a different 17language. On the other hand, if you are using Python and wonder what the precise 18rules about a particular area of the language are, you should definitely be able 19to find them here. If you would like to see a more formal definition of the 20language, maybe you could volunteer your time --- or invent a cloning machine 21:-). 22 23It is dangerous to add too many implementation details to a language reference 24document --- the implementation may change, and other implementations of the 25same language may work differently. On the other hand, there is currently only 26one Python implementation in widespread use (although alternate implementations 27exist), and its particular quirks are sometimes worth being mentioned, 28especially where the implementation imposes additional limitations. Therefore, 29you'll find short "implementation notes" sprinkled throughout the text. 30 31Every Python implementation comes with a number of built-in and standard 32modules. These are documented in :ref:`library-index`. A few built-in modules 33are mentioned when they interact in a significant way with the language 34definition. 35 36 37.. _implementations: 38 39Alternate Implementations 40========================= 41 42Though there is one Python implementation which is by far the most popular, 43there are some alternate implementations which are of particular interest to 44different audiences. 45 46Known implementations include: 47 48CPython 49 This is the original and most-maintained implementation of Python, written in C. 50 New language features generally appear here first. 51 52Jython 53 Python implemented in Java. This implementation can be used as a scripting 54 language for Java applications, or can be used to create applications using the 55 Java class libraries. It is also often used to create tests for Java libraries. 56 More information can be found at `the Jython website <http://www.jython.org/>`_. 57 58Python for .NET 59 This implementation actually uses the CPython implementation, but is a managed 60 .NET application and makes .NET libraries available. It was created by Brian 61 Lloyd. For more information, see the `Python for .NET home page 62 <https://pythonnet.github.io/>`_. 63 64IronPython 65 An alternate Python for .NET. Unlike Python.NET, this is a complete Python 66 implementation that generates IL, and compiles Python code directly to .NET 67 assemblies. It was created by Jim Hugunin, the original creator of Jython. For 68 more information, see `the IronPython website <http://ironpython.net/>`_. 69 70PyPy 71 An implementation of Python written completely in Python. It supports several 72 advanced features not found in other implementations like stackless support 73 and a Just in Time compiler. One of the goals of the project is to encourage 74 experimentation with the language itself by making it easier to modify the 75 interpreter (since it is written in Python). Additional information is 76 available on `the PyPy project's home page <http://pypy.org/>`_. 77 78Each of these implementations varies in some way from the language as documented 79in this manual, or introduces specific information beyond what's covered in the 80standard Python documentation. Please refer to the implementation-specific 81documentation to determine what else you need to know about the specific 82implementation you're using. 83 84 85.. _notation: 86 87Notation 88======== 89 90.. index:: 91 single: BNF 92 single: grammar 93 single: syntax 94 single: notation 95 96The descriptions of lexical analysis and syntax use a modified BNF grammar 97notation. This uses the following style of definition: 98 99.. productionlist:: * 100 name: `lc_letter` (`lc_letter` | "_")* 101 lc_letter: "a"..."z" 102 103The first line says that a ``name`` is an ``lc_letter`` followed by a sequence 104of zero or more ``lc_letter``\ s and underscores. An ``lc_letter`` in turn is 105any of the single characters ``'a'`` through ``'z'``. (This rule is actually 106adhered to for the names defined in lexical and grammar rules in this document.) 107 108Each rule begins with a name (which is the name defined by the rule) and 109``::=``. A vertical bar (``|``) is used to separate alternatives; it is the 110least binding operator in this notation. A star (``*``) means zero or more 111repetitions of the preceding item; likewise, a plus (``+``) means one or more 112repetitions, and a phrase enclosed in square brackets (``[ ]``) means zero or 113one occurrences (in other words, the enclosed phrase is optional). The ``*`` 114and ``+`` operators bind as tightly as possible; parentheses are used for 115grouping. Literal strings are enclosed in quotes. White space is only 116meaningful to separate tokens. Rules are normally contained on a single line; 117rules with many alternatives may be formatted alternatively with each line after 118the first beginning with a vertical bar. 119 120.. index:: 121 single: lexical definitions 122 single: ASCII@ASCII 123 124In lexical definitions (as the example above), two more conventions are used: 125Two literal characters separated by three dots mean a choice of any single 126character in the given (inclusive) range of ASCII characters. A phrase between 127angular brackets (``<...>``) gives an informal description of the symbol 128defined; e.g., this could be used to describe the notion of 'control character' 129if needed. 130 131Even though the notation used is almost the same, there is a big difference 132between the meaning of lexical and syntactic definitions: a lexical definition 133operates on the individual characters of the input source, while a syntax 134definition operates on the stream of tokens generated by the lexical analysis. 135All uses of BNF in the next chapter ("Lexical Analysis") are lexical 136definitions; uses in subsequent chapters are syntactic definitions. 137 138