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<H2><A NAME="SECTION009510000000000000000"></A><A NAME="lock-objects"></A>
<BR>
7.5.1 Lock Objects 
</H2>

<P>
A primitive lock is a synchronization primitive that is not owned
by a particular thread when locked.  In Python, it is currently
the lowest level synchronization primitive available, implemented
directly by the <tt class="module"><a href="module-thread.html">thread</a></tt> extension module.

<P>
A primitive lock is in one of two states, ``locked'' or ``unlocked''.
It is created in the unlocked state.  It has two basic methods,
<tt class="method">acquire()</tt> and <tt class="method">release()</tt>.  When the state is
unlocked, <tt class="method">acquire()</tt> changes the state to locked and returns
immediately.  When the state is locked, <tt class="method">acquire()</tt> blocks
until a call to <tt class="method">release()</tt> in another thread changes it to
unlocked, then the <tt class="method">acquire()</tt> call resets it to locked and
returns.  The <tt class="method">release()</tt> method should only be called in the
locked state; it changes the state to unlocked and returns
immediately.  When more than one thread is blocked in
<tt class="method">acquire()</tt> waiting for the state to turn to unlocked, only one
thread proceeds when a <tt class="method">release()</tt> call resets the state to
unlocked; which one of the waiting threads proceeds is not defined,
and may vary across implementations.

<P>
All methods are executed atomically.

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2720' xml:id='l2h-2720' class="method">acquire</tt></b>(</nobr></td>
  <td><var></var><big>[</big><var>blocking<code> = 1</code></var><big>]</big><var></var>)</td></tr></table></dt>
<dd>
Acquire a lock, blocking or non-blocking.

<P>
When invoked without arguments, block until the lock is
unlocked, then set it to locked, and return true.  

<P>
When invoked with the <var>blocking</var> argument set to true, do the
same thing as when called without arguments, and return true.

<P>
When invoked with the <var>blocking</var> argument set to false, do not
block.  If a call without an argument would block, return false
immediately; otherwise, do the same thing as when called
without arguments, and return true.
</dl>

<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
  <td><nobr><b><tt id='l2h-2721' xml:id='l2h-2721' class="method">release</tt></b>(</nobr></td>
  <td><var></var>)</td></tr></table></dt>
<dd>
Release a lock.

<P>
When the lock is locked, reset it to unlocked, and return.  If
any other threads are blocked waiting for the lock to become
unlocked, allow exactly one of them to proceed.

<P>
Do not call this method when the lock is unlocked.

<P>
There is no return value.
</dl>

<P>

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	55	<H2><A NAME="SECTION009510000000000000000"></A><A NAME="lock-objects"></A>
	56	<BR>
	57	7.5.1 Lock Objects
	58	</H2>
	59
	60	<P>
	61	A primitive lock is a synchronization primitive that is not owned
	62	by a particular thread when locked. In Python, it is currently
	63	the lowest level synchronization primitive available, implemented
	64	directly by the <tt class="module"><a href="module-thread.html">thread</a></tt> extension module.
65
66	<P>
67	A primitive lock is in one of two states, ``locked'' or ``unlocked''.
68	It is created in the unlocked state. It has two basic methods,
69	<tt class="method">acquire()</tt> and <tt class="method">release()</tt>. When the state is
70	unlocked, <tt class="method">acquire()</tt> changes the state to locked and returns
71	immediately. When the state is locked, <tt class="method">acquire()</tt> blocks
72	until a call to <tt class="method">release()</tt> in another thread changes it to
73	unlocked, then the <tt class="method">acquire()</tt> call resets it to locked and
74	returns. The <tt class="method">release()</tt> method should only be called in the
75	locked state; it changes the state to unlocked and returns
76	immediately. When more than one thread is blocked in
77	<tt class="method">acquire()</tt> waiting for the state to turn to unlocked, only one
78	thread proceeds when a <tt class="method">release()</tt> call resets the state to
79	unlocked; which one of the waiting threads proceeds is not defined,
80	and may vary across implementations.
81
82	<P>
83	All methods are executed atomically.
84
85	<P>
86	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
87	<td><nobr><b><tt id='l2h-2720' xml:id='l2h-2720' class="method">acquire</tt></b>(</nobr></td>
88	<td><var></var><big>[</big><var>blocking<code> = 1</code></var><big>]</big><var></var>)</td></tr></table></dt>
89	<dd>
90	Acquire a lock, blocking or non-blocking.
91
92	<P>
93	When invoked without arguments, block until the lock is
94	unlocked, then set it to locked, and return true.
95
96	<P>
97	When invoked with the <var>blocking</var> argument set to true, do the
98	same thing as when called without arguments, and return true.
99
100	<P>
101	When invoked with the <var>blocking</var> argument set to false, do not
102	block. If a call without an argument would block, return false
103	immediately; otherwise, do the same thing as when called
104	without arguments, and return true.
105	</dl>
106
107	<P>
108	<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline">
109	<td><nobr><b><tt id='l2h-2721' xml:id='l2h-2721' class="method">release</tt></b>(</nobr></td>
110	<td><var></var>)</td></tr></table></dt>
111	<dd>
112	Release a lock.
113
114	<P>
115	When the lock is locked, reset it to unlocked, and return. If
116	any other threads are blocked waiting for the lock to become
117	unlocked, allow exactly one of them to proceed.
118
119	<P>
120	Do not call this method when the lock is unlocked.
121
122	<P>
123	There is no return value.
124	</dl>
125
126	<P>
127
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