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| 55 | <H1><A NAME="SECTION005100000000000000000"></A><A NAME="objects"></A><a id='l2h-19' xml:id='l2h-19'></a> |
| 56 | <BR> |
| 57 | 3.1 Objects, values and types |
| 58 | </H1> |
| 59 | |
| 60 | <P> |
| 61 | <i class="dfn">Objects</i> are Python's abstraction for data. All data in a Python |
| 62 | program is represented by objects or by relations between objects. |
| 63 | (In a sense, and in conformance to Von Neumann's model of a |
| 64 | ``stored program computer,'' code is also represented by objects.) |
| 65 | |
| 66 | <P> |
| 67 | Every object has an identity, a type and a value. An object's |
| 68 | <em>identity</em> never changes once it has been created; you may think |
| 69 | of it as the object's address in memory. The `<tt class="keyword">is</tt>' operator |
| 70 | compares the identity of two objects; the |
| 71 | <tt class="function">id()</tt><a id='l2h-20' xml:id='l2h-20'></a> function returns an integer |
| 72 | representing its identity (currently implemented as its address). |
| 73 | An object's <i class="dfn">type</i> is |
| 74 | also unchangeable.<A NAME="tex2html2" |
| 75 | HREF="#foot2364"><SUP>3.1</SUP></A>An object's type determines the operations that the object |
| 76 | supports (e.g., ``does it have a length?'') and also defines the |
| 77 | possible values for objects of that type. The |
| 78 | <tt class="function">type()</tt><a id='l2h-21' xml:id='l2h-21'></a> function returns an object's type |
| 79 | (which is an object itself). The <em>value</em> of some |
| 80 | objects can change. Objects whose value can change are said to be |
| 81 | <em>mutable</em>; objects whose value is unchangeable once they are |
| 82 | created are called <em>immutable</em>. |
| 83 | (The value of an immutable container object that contains a reference |
| 84 | to a mutable object can change when the latter's value is changed; |
| 85 | however the container is still considered immutable, because the |
| 86 | collection of objects it contains cannot be changed. So, immutability |
| 87 | is not strictly the same as having an unchangeable value, it is more |
| 88 | subtle.) |
| 89 | An object's mutability is determined by its type; for instance, |
| 90 | numbers, strings and tuples are immutable, while dictionaries and |
| 91 | lists are mutable. |
| 92 | |
| 93 | <P> |
| 94 | Objects are never explicitly destroyed; however, when they become |
| 95 | unreachable they may be garbage-collected. An implementation is |
| 96 | allowed to postpone garbage collection or omit it altogether -- it is |
| 97 | a matter of implementation quality how garbage collection is |
| 98 | implemented, as long as no objects are collected that are still |
| 99 | reachable. (Implementation note: the current implementation uses a |
| 100 | reference-counting scheme with (optional) delayed detection of |
| 101 | cyclically linked garbage, which collects most objects as soon as they |
| 102 | become unreachable, but is not guaranteed to collect garbage |
| 103 | containing circular references. See the |
| 104 | <em class="citetitle"><a |
| 105 | href="../lib/module-gc.html" |
| 106 | title="Python Library Reference" |
| 107 | >Python Library Reference</a></em> for |
| 108 | information on controlling the collection of cyclic garbage.) |
| 109 | |
| 110 | <P> |
| 111 | Note that the use of the implementation's tracing or debugging |
| 112 | facilities may keep objects alive that would normally be collectable. |
| 113 | Also note that catching an exception with a |
| 114 | `<tt class="keyword">try</tt>...<tt class="keyword">except</tt>' statement may keep objects alive. |
| 115 | |
| 116 | <P> |
| 117 | Some objects contain references to ``external'' resources such as open |
| 118 | files or windows. It is understood that these resources are freed |
| 119 | when the object is garbage-collected, but since garbage collection is |
| 120 | not guaranteed to happen, such objects also provide an explicit way to |
| 121 | release the external resource, usually a <tt class="method">close()</tt> method. |
| 122 | Programs are strongly recommended to explicitly close such |
| 123 | objects. The `<tt class="keyword">try</tt>...<tt class="keyword">finally</tt>' statement provides |
| 124 | a convenient way to do this. |
| 125 | |
| 126 | <P> |
| 127 | Some objects contain references to other objects; these are called |
| 128 | <em>containers</em>. Examples of containers are tuples, lists and |
| 129 | dictionaries. The references are part of a container's value. In |
| 130 | most cases, when we talk about the value of a container, we imply the |
| 131 | values, not the identities of the contained objects; however, when we |
| 132 | talk about the mutability of a container, only the identities of |
| 133 | the immediately contained objects are implied. So, if an immutable |
| 134 | container (like a tuple) |
| 135 | contains a reference to a mutable object, its value changes |
| 136 | if that mutable object is changed. |
| 137 | |
| 138 | <P> |
| 139 | Types affect almost all aspects of object behavior. Even the importance |
| 140 | of object identity is affected in some sense: for immutable types, |
| 141 | operations that compute new values may actually return a reference to |
| 142 | any existing object with the same type and value, while for mutable |
| 143 | objects this is not allowed. E.g., after |
| 144 | "<tt class="samp">a = 1; b = 1</tt>", |
| 145 | <code>a</code> and <code>b</code> may or may not refer to the same object with the |
| 146 | value one, depending on the implementation, but after |
| 147 | "<tt class="samp">c = []; d = []</tt>", <code>c</code> and <code>d</code> |
| 148 | are guaranteed to refer to two different, unique, newly created empty |
| 149 | lists. |
| 150 | (Note that "<tt class="samp">c = d = []</tt>" assigns the same object to both |
| 151 | <code>c</code> and <code>d</code>.) |
| 152 | |
| 153 | <P> |
| 154 | <BR><HR><H4>Footnotes</H4> |
| 155 | <DL> |
| 156 | <DT><A NAME="foot2364">... unchangeable.</A><A |
| 157 | href="objects.html#tex2html2"><SUP>3.1</SUP></A></DT> |
| 158 | <DD>Since Python 2.2, a gradual merging of |
| 159 | types and classes has been started that makes this and a few other |
| 160 | assertions made in this manual not 100% accurate and complete: |
| 161 | for example, it <em>is</em> now possible in some cases to change an |
| 162 | object's type, under certain controlled conditions. Until this manual |
| 163 | undergoes extensive revision, it must now be taken as authoritative |
| 164 | only regarding ``classic classes'', that are still the default, for |
| 165 | compatibility purposes, in Python 2.2 and 2.3. |
| 166 | |
| 167 | </DD> |
| 168 | </DL> |
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| 203 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> |
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