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| 17 | <title>3.1 Objects, values and types</title> | |
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| 54 | ||
| 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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