1 \section{\module{thread
} ---
2 Multiple threads of control
}
4 \declaremodule{builtin
}{thread
}
5 \modulesynopsis{Create multiple threads of control within one interpreter.
}
8 This module provides low-level primitives for working with multiple
9 threads (a.k.a.\
\dfn{light-weight processes
} or
\dfn{tasks
}) --- multiple
10 threads of control sharing their global data space. For
11 synchronization, simple locks (a.k.a.\
\dfn{mutexes
} or
\dfn{binary
12 semaphores
}) are provided.
13 \index{light-weight processes
}
14 \index{processes, light-weight
}
15 \index{binary semaphores
}
16 \index{semaphores, binary
}
18 The module is optional. It is supported on Windows, Linux, SGI
19 IRIX, Solaris
2.x, as well as on systems that have a
\POSIX{} thread
20 (a.k.a. ``pthread'') implementation. For systems lacking the
\module{thread
}
21 module, the
\refmodule[dummythread
]{dummy_thread
} module is available.
22 It duplicates this module's interface and can be
23 used as a drop-in replacement.
25 \indexii{threads
}{\POSIX}
27 It defines the following constant and functions:
29 \begin{excdesc
}{error
}
30 Raised on thread-specific errors.
33 \begin{datadesc
}{LockType
}
34 This is the type of lock objects.
37 \begin{funcdesc
}{start_new_thread
}{function, args
\optional{, kwargs
}}
38 Start a new thread and return its identifier. The thread executes the function
39 \var{function
} with the argument list
\var{args
} (which must be a tuple). The
40 optional
\var{kwargs
} argument specifies a dictionary of keyword arguments.
41 When the function returns, the thread silently exits. When the function
42 terminates with an unhandled exception, a stack trace is printed and
43 then the thread exits (but other threads continue to run).
46 \begin{funcdesc
}{interrupt_main
}{}
47 Raise a
\exception{KeyboardInterrupt
} exception in the main thread. A subthread
48 can use this function to interrupt the main thread.
52 \begin{funcdesc
}{exit
}{}
53 Raise the
\exception{SystemExit
} exception. When not caught, this
54 will cause the thread to exit silently.
57 %\begin{funcdesc}{exit_prog}{status}
58 %Exit all threads and report the value of the integer argument
59 %\var{status} as the exit status of the entire program.
60 %\strong{Caveat:} code in pending \keyword{finally} clauses, in this thread
61 %or in other threads, is not executed.
64 \begin{funcdesc
}{allocate_lock
}{}
65 Return a new lock object. Methods of locks are described below. The
66 lock is initially unlocked.
69 \begin{funcdesc
}{get_ident
}{}
70 Return the `thread identifier' of the current thread. This is a
71 nonzero integer. Its value has no direct meaning; it is intended as a
72 magic cookie to be used e.g. to index a dictionary of thread-specific
73 data. Thread identifiers may be recycled when a thread exits and
74 another thread is created.
77 \begin{funcdesc
}{stack_size
}{\optional{size
}}
78 Return the thread stack size used when creating new threads. The
79 optional
\var{size
} argument specifies the stack size to be used for
80 subsequently created threads, and must be
0 (use platform or
81 configured default) or a positive integer value of at least
32,
768 (
32kB).
82 If changing the thread stack size is unsupported, a
\exception{ThreadError
}
83 is raised. If the specified stack size is invalid, a
\exception{ValueError
}
84 is raised and the stack size is unmodified.
32kB is currently the minimum
85 supported stack size value to guarantee sufficient stack space for the
86 interpreter itself. Note that some platforms may have particular
87 restrictions on values for the stack size, such as requiring a minimum
88 stack size >
32kB or requiring allocation in multiples of the system
89 memory page size - platform documentation should be referred to for
90 more information (
4kB pages are common; using multiples of
4096 for
91 the stack size is the suggested approach in the absence of more
92 specific information).
93 Availability: Windows, systems with
\POSIX{} threads.
98 Lock objects have the following methods:
100 \begin{methoddesc
}[lock
]{acquire
}{\optional{waitflag
}}
101 Without the optional argument, this method acquires the lock
102 unconditionally, if necessary waiting until it is released by another
103 thread (only one thread at a time can acquire a lock --- that's their
104 reason for existence). If the integer
105 \var{waitflag
} argument is present, the action depends on its
106 value: if it is zero, the lock is only acquired if it can be acquired
107 immediately without waiting, while if it is nonzero, the lock is
108 acquired unconditionally as before. The
109 return value is
\code{True
} if the lock is acquired successfully,
113 \begin{methoddesc
}[lock
]{release
}{}
114 Releases the lock. The lock must have been acquired earlier, but not
115 necessarily by the same thread.
118 \begin{methoddesc
}[lock
]{locked
}{}
119 Return the status of the lock:\
\code{True
} if it has been acquired by
120 some thread,
\code{False
} if not.
123 In addition to these methods, lock objects can also be used via the
124 \keyword{with
} statement, e.g.:
127 from __future__ import with_statement
130 a_lock = thread.allocate_lock()
133 print "a_lock is locked while this executes"
140 Threads interact strangely with interrupts: the
141 \exception{KeyboardInterrupt
} exception will be received by an
142 arbitrary thread. (When the
\refmodule{signal
}\refbimodindex{signal
}
143 module is available, interrupts always go to the main thread.)
146 Calling
\function{sys.exit()
} or raising the
\exception{SystemExit
}
147 exception is equivalent to calling
\function{exit()
}.
150 Not all built-in functions that may block waiting for I/O allow other
151 threads to run. (The most popular ones (
\function{time.sleep()
},
152 \method{\var{file
}.read()
},
\function{select.select()
}) work as
156 It is not possible to interrupt the
\method{acquire()
} method on a lock
157 --- the
\exception{KeyboardInterrupt
} exception will happen after the
158 lock has been acquired.
161 When the main thread exits, it is system defined whether the other
162 threads survive. On SGI IRIX using the native thread implementation,
163 they survive. On most other systems, they are killed without
164 executing
\keyword{try
} ...
\keyword{finally
} clauses or executing
166 \indexii{threads
}{IRIX
}
169 When the main thread exits, it does not do any of its usual cleanup
170 (except that
\keyword{try
} ...
\keyword{finally
} clauses are honored),
171 and the standard I/O files are not flushed.