Cpselect python

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Cpselect python

A network socket is an endpoint of an interprocess communication across a computer network. The Python Standard Library has a module called socket which provides a low-level internet networking interface.

This interface is common across different programming languages since it uses OS-level system calls. To create a socket, there is a function called socket. It accepts familytypeand proto arguments see documentation for details.

cpselect python

To create a TCP-socket, you should use socket. Here is an example of Echo server from documentation:. Here we create a server socket, bind it to a localhost and port, and start listening for incoming connections. To accept an incoming connection we call accept method which will block until a new client connects. When this happens, it creates a new socket and returns it together with the client's address.

Then, in an infinite cycle, it reads data from the socket in batches of bytes using method recv until it returns an empty string. After that, it sends all incoming data back using a convenient method sendall which inside repeatedly calls send. And after that it simply closes the client's connection. This example can serve only one incoming connection because it does not call accept in a cycle. Here instead of bind and listen it calls only connect and immediately sends data to the server.

Then it receives bytes back, closes the socket, and prints the received data. All socket methods are blocking. For example, when it reads from a socket or writes to it the program can't do anything else. One possible solution is to delegate working with clients to separate threads.

However, creating threads and switching contexts between them is not really a cheap operation. To address this problem, there is a so-called asynchronous way of working with sockets. The main idea is to delegate maintaining the socket's state to an operating system and letting it notify the program when there is something to read from the socket or when it is ready for writing. As you can see, there is much more code than in the blocking Echo server.

That is primarily because we have to maintain a set of queues for different lists of sockets, i. Creating server socket looks the same except for one line: server. This is done to make the socket nonblocking. This server is more advanced since it can serve more than one client. The main point is in selecting sockets:. Here we call select. That is why it passes three lists of sockets to specify which socket is expected to be writable, readable, and which should be checked for errors.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

The dark mode beta is finally here. Change your preferences any time. Stack Overflow for Teams is a private, secure spot for you and your coworkers to find and share information. So the question is: how should the python code be modified to select those fooid s where bar is in 'A','C'?

By the way, I noticed that if I switch the roles of bar and fooidI can get the code to select those bar s where fooid is in 1,3 successfully.

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I don't understand why one such query below works, while the other one above doesn't. Thanks to Amber's comment, I understand better what is going wrong. Or must one manually construct the SQL string? You can leave any chars you want in the string, no need to remove or quote chars. Unfortunately, you need to manually construct the query parameters, because as far as I know, there is no built-in bind method for binding a list to an IN clause, similar to Hibernate's setParameterList.

However, you can accomplish the same with the following:. If you have other parameters in the query, beyond the IN list, then the following extension to JG's answer may be useful.

A better name would be nice, though For Python3, use str instead of basestring. Learn more. Asked 9 years, 3 months ago. Active 2 years, 1 month ago. Viewed k times. Can you look in your MySQL server's logs and find out what query was actually run? Active Oldest Votes. Robert Bergs Robert Bergs 5 5 silver badges 8 8 bronze badges. I like this, I would go as far as saying that the accepted answer is to be considered harmful!This module provides access to the select and poll functions available in most operating systems, epoll available on Linux 2.

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Note that on Windows, it only works for sockets; on other operating systems, it also works for other file types in particular, on Unix, it works on pipes. It cannot be used on regular files to determine whether a file has grown since it was last read.

The exception raised when an error occurs. The accompanying value is a pair containing the numeric error code from errno and the corresponding string, as would be printed by the C function perror.

Only supported on Linux 2. Not supported by all operating systems. Only supported on BSD. Returns a kernel queue object; see section Kqueue Objects below for the methods supported by kqueue objects. Returns a kernel event object; see section Kevent Objects below for the methods supported by kevent objects.

This is a straightforward interface to the Unix select system call. Empty sequences are allowed, but acceptance of three empty sequences is platform-dependent. It is known to work on Unix but not on Windows. The optional timeout argument specifies a time-out as a floating point number in seconds. When the timeout argument is omitted the function blocks until at least one file descriptor is ready.

A time-out value of zero specifies a poll and never blocks. The return value is a triple of lists of objects that are ready: subsets of the first three arguments. When the time-out is reached without a file descriptor becoming ready, three empty lists are returned.

Among the acceptable object types in the sequences are Python file objects e. You may also define a wrapper class yourself, as long as it has an appropriate fileno method that really returns a file descriptor, not just a random integer.

File objects on Windows are not acceptable, but sockets are. Availability: Unix.

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The poll system call, supported on most Unix systems, provides better scalability for network servers that service many, many clients at the same time. Register a file descriptor with the polling object.

File objects implement filenoso they can also be used as the argument. If not specified, the default value used will check for all 3 types of events. Modifies an already registered fd. This has the same effect as register fd, eventmask. Remove a file descriptor being tracked by a polling object. Just like the register method, fd can be an integer or an object with a fileno method that returns an integer. Attempting to remove a file descriptor that was never registered causes a KeyError exception to be raised.

Polls the set of registered file descriptors, and returns a possibly-empty list containing fd, event 2-tuples for the descriptors that have events or errors to report. An empty list indicates that the call timed out and no file descriptors had any events to report.

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If timeout is given, it specifies the length of time in milliseconds which the system will wait for events before returning. If timeout is omitted, negative, or Nonethe call will block until there is an event for this poll object. Value used to identify the event. In the constructor ident can either be an int or an object with a fileno function. Optional Operating System Services. The module defines the following: exception select.

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Why this ad?This module provides access to the select and poll functions available in most operating systems, devpoll available on Solaris and derivatives, epoll available on Linux 2.

Note that on Windows, it only works for sockets; on other operating systems, it also works for other file types in particular, on Unix, it works on pipes. It cannot be used on regular files to determine whether a file has grown since it was last read. Users are encouraged to use the selectors module instead, unless they want precise control over the OS-level primitives used. A deprecated alias of OSError. Changed in version 3. Only supported on Solaris and derivatives.

If your program reduces this value, devpoll will fail. If your program increases this value, devpoll may return an incomplete list of active file descriptors. The new file descriptor is non-inheritable. Only supported on Linux 2. It must be positive, or -1 to use the default. However, when supplied, its value must be 0 or select.

See the Edge and Level Trigger Polling epoll Objects section below for the methods supported by epolling objects.

The new file descriptor is now non-inheritable. Deprecated since version 3. Use os. Not supported by all operating systems. Only supported on BSD. Returns a kernel queue object; see section Kqueue Objects below for the methods supported by kqueue objects. Returns a kernel event object; see section Kevent Objects below for the methods supported by kevent objects.

cpselect python

This is a straightforward interface to the Unix select system call. Empty sequences are allowed, but acceptance of three empty sequences is platform-dependent. It is known to work on Unix but not on Windows. The optional timeout argument specifies a time-out as a floating point number in seconds.

When the timeout argument is omitted the function blocks until at least one file descriptor is ready. A time-out value of zero specifies a poll and never blocks. The return value is a triple of lists of objects that are ready: subsets of the first three arguments. When the time-out is reached without a file descriptor becoming ready, three empty lists are returned. Among the acceptable object types in the sequences are Python file objects e.

You may also define a wrapper class yourself, as long as it has an appropriate fileno method that really returns a file descriptor, not just a random integer. File objects on Windows are not acceptable, but sockets are. The minimum number of bytes which can be written without blocking to a pipe when the pipe has been reported as ready for writing by selectpoll or another interface in this module.Some of the features described here may not be available in earlier versions of Python.

Now available for Python 3! Buy the book! The module also includes polla Unix-only API, and several options that only work with specific variants of Unix. It monitors sockets, open files, and pipes anything with a fileno method that returns a valid file descriptor until they become readable or writable, or a communication error occurs. The echo server example from the socket section can be extended to watch for more than one connection at a time by using select.

The arguments to select are three lists containing communication channels to monitor. The first is a list of the objects to be checked for incoming data to be read, the second contains objects that will receive outgoing data when there is room in their buffer, and the third those that may have an error usually a combination of the input and output channel objects.

The next step in the server is to set up the lists containing input sources and output destinations to be passed to select. Connections are added to and removed from these lists by the server main loop. Since this version of the server is going to wait for a socket to become writable before sending any data instead of immediately sending the replyeach output connection needs a queue to act as a buffer for the data to be sent through it.

The main portion of the server program loops, calling select to block and wait for network activity. All of the sockets in the readable list have incoming data buffered and available to be read.

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All of the sockets in the writable list have free space in their buffer and can be written to. In addition to adding the new connection to the list of inputs to monitor, this section sets the client socket to not block. The next case is an established connection with a client that has sent data. The data is read with recvthen placed on the queue so it can be sent through the socket and back to the client. A readable socket without data available is from a client that has disconnected, and the stream is ready to be closed.

There are fewer cases for the writable connections. If there is data in the queue for a connection, the next message is sent.

“How to Work with TCP Sockets in Python (with Select Example)”

Otherwise, the connection is removed from the list of output connections so that the next time through the loop select does not indicate that the socket is ready to send data. The example client program uses two sockets to demonstrate how the server with select manages multiple connections at the same time. Then it sends one pieces of the message at a time via each socket, and reads all responses available after writing new data. Run the server in one window and the client in another.

The output will look like this, with different port numbers. Using a timeout value lets a main program call select as part of a larger processing loop, taking other actions in between checking for network input. When the timeout expires, select returns three empty lists. Updating the server example to use a timeout requires adding the extra argument to the select call and handling the empty lists after select returns.

The poll function provides similar features to selectbut the underlying implementation is more efficient. The trade-off is that poll is not supported under Windows, so programs using poll are less portable. An echo server built on poll starts with the same socket configuration code used in the other examples. The timeout value passed to poll is represented in milliseconds, instead of seconds, so in order to pause for a full second the timeout must be set to Python implements poll with a class that manages the registered data channels being monitored.

Channels are added by calling register with flags indicating which events are interesting for that channel. The full set of flags is:. The echo server will be setting up some sockets just for reading, and others to be read from or written to. The server socket is registered so that any incoming connections or data triggers an event. Since poll returns a list of tuples containing the file descriptor for the socket and the event flag, a mapping from file descriptor numbers to objects is needed to retrieve the socket to read or write from it.

Sockets other than the server are existing clients, and recv is used to access the data waiting to be read.GitHub is home to over 40 million developers working together to host and review code, manage projects, and build software together.

cpselect python

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cpselect python

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