Advanced Asynchronous IO Example

The select module is sometimes used to implement servers based on tasklets or coroutines a technique that can be used to provide concurrent execution without threads or processes. The following advanced example illustrates this concept by implementing an I O-based task scheduler for coroutines. Be forewarned this is the most advanced example in the book and it will require some study for it to make sense.You might also want to consult my PyCON'09 tutorial A Curious Course on Coroutines and...

Advanced Metaclasses

In Python 2, you can define metaclasses that alter the behavior of classes. A subtle facet of the implementation is that the processing carried out by a metaclass only occurs after the body of a class has executed. That is, the interpreter executes the entire body of a class and populates a dictionary. Once the dictionary has been populated, the dictionary is passed to the metaclass constructor (after the body of the class has executed). In Python 3, metaclasses can additionally carry out extra...

An Extension Module Prototype

Extension modules are built by writing a separate C source file that contains a set of wrapper functions which provide the glue between the Python interpreter and the underlying C code. Here is an example of a basic extension module called _example include Python.h include example.h static char py_gcd_doc Computes the GCD of two integers static PyObject * py_gcd(PyObject *self, PyObject *args) int x,y,r return Py_BuildValue(i,r) static char py_replace_doc Replaces all characters in a string...

Buffered Binary IO

The buffered I O layer contains a collection of file objects that read and write raw binary data, but with in-memory buffering. As input, these objects all require a file object that implements raw I O such as the FileIO object in the previous section. All of the classes in this section inherit from BufferedIOBase. A class for buffered binary reading on a raw file specified in raw. buffer_size specifies the buffer size to use in bytes. If omitted, the value of DEFAULT_BUFFER_SIZE is used (8,192...

Builtin Types for Representing Data

There are approximately a dozen built-in data types that are used to represent most of the data used in programs. These are grouped into a few major categories as shown in Table 3.1.The Type Name column in the table lists the name or expression that you can use to check for that type using isinstance() and other type-related functions. Certain types are only available in Python 2 and have been indicated as such (in Python 3, they have been deprecated or merged into one of the other types)....

CGI Programming Advice

In the current age of web frameworks, CGI scripting seems to have fallen out of fashion. However, if you are going to use it, there are a couple of programming tips that can simplify your life. First, don't write CGI scripts where you are using a huge number of print statements to produce hard-coded HTML output. The resulting program will be a horrible tangled mess of Python and HTML that is not only impossible to read, but also impossible to maintain. A better approach is to rely on templates....

Classes Types and Instances

When you define a class, the class definition normally produces an object of type type. Here's an example > > > class Foo(object) pass The following table shows commonly used attributes of a type object t Dictionary holding class methods and variables Module name in which the class is defined Set of abstract method names (may be undefined if there aren't any) When an object instance is created, the type of the instance is the class that defined it. Here's an example > > > f Foo()...

Composing Email

To compose an email message, you can either create an empty instance of a Message object, which is defined in the email.message module, or you can use a Message object that was created by parsing an email message (see the previous section). Creates a new message that is initially empty. An instance m of Message supports the following methods for populating a message with content, headers, and other information. Adds a new message header. name is the name of the header, value is the value of the...

Connection Objects

The Connection object c returned by the connect() function supports the standard operations described in the Database API. In addition, the following methods specific to the sqlite3 module are provided. c.create_function(name, num_params, func) Creates a user-defined function that can be used in SQL statements. name is a string containing the name of the function, num_params is an integer giving the number of parameters, and func is a Python function that provides the implementation. Here is a...

Context Managers and the with Statement

Proper management of system resources such as files, locks, and connections is often a tricky problem when combined with exceptions. For example, a raised exception can cause control flow to bypass statements responsible for releasing critical resources such as a lock. The with statement allows a series of statements to execute inside a runtime context that is controlled by an object that serves as a context manager. Here is an example with as f f.write(Debugging n) statements f.write(Done n)...

Cursors and Basic Operations

To perform basic operations on a sqlite3 database, you first have to create a cursor object using the cursor() method of a connection.You then use the execute(), executemany(), or executescript() methods of the cursor to execute SQL statements. See the Database API section for further details about the general operation of these methods. Instead of repeating that information here, a set of common database use cases are presented along with sample code. The goal is to show both the operation of...

Data Encapsulation and Private Attributes

By default, all attributes and methods of a class are public.This means that they are all accessible without any restrictions. It also implies that everything defined in a base class is inherited and accessible within a derived class. This behavior is often undesirable in object-oriented applications because it exposes the internal implementation of an object and can lead to namespace conflicts between objects defined in a derived class and those defined in a base class. To fix this problem,...

DBMStyle Database Modules

Python includes a number of library modules for supporting UNIX DBM-style database files. Several standard types of these databases are supported. The dbm module is used to read standard UNIX-dbm database files. The gdbm module is used to read GNU dbm database files (http www.gnu.org software gdbm).The dbhash module is used to read database files created by the Berkeley DB library (http www.oracle.com database berkeley-db index.html).The dumbdbm module is a pure-Python module that implements a...

Del s[ij strideDeletes an extended slice

The s i x operator changes element i of a list to refer to object x, increasing the reference count of x. Negative indices are relative to the end of the list, and attempts to assign a value to an out-of-range index result in an IndexError exception.The slicing assignment operator s i j r replaces element k, where i < k < j,with elements from sequence r. Indices may have the same values as for slicing and are adjusted to the beginning or end of the list if they're out of range. If...

Documentation Strings and the doctest Module

If the first line of a function, class, or module is a string, that string is known as a documentation string. The inclusion of documentation strings is considered good style because these strings are used to supply information to Python software development tools. For example, the help() command inspects documentation strings, and Python IDEs look at the strings as well. Because programmers tend to view documentation strings while experimenting in the interactive shell, it is common for the...

File and Directory Handling

His chapter describes Python modules for high-level file and directory handling. Topics include modules for processing various kinds of basic file encodings such as gzip and bzip2 files, modules for extracting file archives such as zip and tar files, and modules for manipulating the file system itself (e.g., directory listings, moving, renaming, copying, and so on). Low-level operating system calls related to files are covered in Chapter 19,Operating System Services. Modules for parsing the...

Functions as Objects and Closures

Functions are first-class objects in Python.This means that they can be passed as arguments to other functions, placed in data structures, and returned by a function as a result. Here is an example of a function that accepts another function as input and calls it Here is an example of using the above function > > > foo.callf(helloworld) Pass a function as an argument When a function is handled as data, it implicitly carries information related to the surrounding environment where the...

Installing Third Party Libraries

The definitive resource for locating third-party libraries and extensions to Python is the Python Package Index (PyPI), which is located at http pypi.python.org. Installing third-party modules is usually straightforward but can become quite involved for very large packages that also depend on other third-party modules. For the more major extensions, you will often find a platform-native installer that simply steps you through the process using a series of dialog screens. For other modules, you...

Internet Application Programming

His chapter describes modules related to Internet application protocols including HTTP, XML-RPC, FTP, and SMTP. Web programming topics such as CGI scripting are covered in Chapter 23,Web Programming. Modules related to dealing with common Internet-related data formats are covered in Chapter 24,Internet Data Handling and Encoding. The organization of network-related library modules is one area where there are significant differences between Python 2 and 3. In the interest of looking forward,...

Interpreter Options and Environment

The interpreter has a number of options that control its runtime behavior and environment. Options are given to the interpreter on the command line as follows python options -c cmd filename - args Here's a list of the most common command-line options Table 10.1 Interpreter Command-Line Arguments Enables warnings about features that are being removed or changed in Python 3. Prevents the creation of .pyc or .pyo files on import. Ignores environment variables. Prints a list of all available...

Iteration and Looping

The most widely used looping construct is the for statement, which is used to iterate over a collection of items. Iteration is one of Python's richest features. However, the most common form of iteration is to simply loop over all the members of a sequence such as a string, list, or tuple. Here's an example print 2 to the d power is d (n, 2**n) In this example, the variable n will be assigned successive items from the list 1,2,3,4, ,9 on each iteration. Because looping over ranges of integers...

Jython and Iron Python

Extending and embedding is not restricted to C programs. If you are working with Java, consider the use ofJython (http www.jython.org), a complete reimplementation of the Python interpreter in Java.With jython, you can simply import Java libraries using the import statement. For example Type copyright, credits or license for more information. > > > from java.lang import System > > > System.out.println(Hello World) If you are working with the .NET framework on Windows, consider the...

Launching Python Applications

In most cases, you'll want programs to start the interpreter automatically, rather than first having to start the interpreter manually. On UNIX, this is done by giving the program execute permission and setting the first line of a program to something like this usr bin env python Python code from this point on print Hello world On Windows, double-clicking a .py, .pyw, .wpy, .pyc, or .pyo file automatically launches the interpreter. Normally, programs run in a console window unless they're...

Loops and Iteration

You implement loops using the for and while statements. Here's an example The while statement executes statements until the associated expression evaluates to false. The for statement iterates over all the elements of s until no more elements are available.The for statement works with any object that supports iteration.This obviously includes the built-in sequence types such as lists, tuples, and strings, but also any object that implements the iterator protocol. An object, s, supports...

Low Level codecs Interface

Each character encoding is assigned a common name such as 'utf-8' or 'big5'. The following function is used to perform a lookup. Looks up a codec in the codec registry. encoding is a string such as 'utf-8'. If nothing is known about the requested encoding, LookupError is raised. Otherwise, an instance c of CodecInfo is returned. A CodecInfo instance c has the following methods A stateless encoding function that encodes the Unicode string s and returns a tuple (bytes, length_consumed). bytes is...

Module Level Functions

The following functions are defined by the sqlite3 module connect(database , timeout , isolation_level , detect_types ) Creates a connection to a SQLite database. database is a string that specifies the name of the database file. It can also be a string memory , in which case an in-memory database is used (note that this kind of database only persists as long as the Python process remains running and would be lost on program exit).The timeout parameter specifies the amount of time to wait for...

Network Programming Basics

Python's network programming modules primarily support two Internet protocols TCP and UDP. The TCP protocol is a reliable connection-oriented protocol used to establish a two-way communications stream between machines. UDP is a lower-level packet-based protocol (connectionless) in which machines send and receive discrete packets of information without formally establishing a connection. Unlike TCP, UDP communication is unreliable and thus inherently more complicated to manage in applications...

Notes

Calling the close() method of a GzipFile object doesn't close files passed in fileobj.This allows additional information to be written to a file after the compressed data. Files produced by the UNIX compress program are not supported. This module requires the zlib module. The shutil module is used to perform high-level file operations such as copying, removing, and renaming. The functions in this module should only be used for proper files and directories. In particular, they do not work for...

Object Behavior and Special Methods

Objects in Python are generally classified according to their behaviors and the features that they implement. For example, all of the sequence types such as strings, lists, and tuples are grouped together merely because they all happen to support a common set of sequence operations such as s n , len(s), etc. All basic interpreter operations are implemented through special object methods.The names of special methods are always preceded and followed by double underscores (__).These methods are...

Object Creation and Destruction

The methods in Table 3.11 create, initialize, and destroy instances.__new__() is a class method that is called to create an instance.The__init__() method initializes the attributes of an object and is called immediately after an object has been newly created. The__del__() method is invoked when an object is about to be destroyed.This method is invoked only when an object is no longer in use. It's important to note that the statement del x only decrements an object's reference count and doesn't...

Object Memory Management

The creation of an instance is carried out in two steps using the special method __new__() , which creates a new instance, and __init__() , which initializes it. For example, the operation c Circle(4.0) performs these steps c Circle.__new__(Circle, 4.0) if isinstance(c,Circle) Circle.__init__(c,4.0) The __new__() method of a class is something that is rarely defined by user code. If it is defined, it is typically written with the prototype __new__(cls, *args, **kwargs) where args and kwargs are...

Operations on Numbers

The following operations can be applied to all numeric types The truncating division operator ( , also known as floor division) truncates the result to an integer and works with both integers and floating-point numbers. In Python 2, the true division operator ( ) also truncates the result to an integer if the operands are inte-gers.Therefore, 7 4 is 1, not 1.75. However, this behavior changes in Python 3, where division produces a floating-point result. The modulo operator returns the remainder...

Other self Python only

The methods__iadd__(),__isub__(), and so forth are used to support in-place arithmetic operators such as a+ b and a- b (also known as augmented assignment). A distinction is made between these operators and the standard arithmetic methods because the implementation of the in-place operators might be able to provide certain customization such as performance optimizations. For instance, if the self parameter is not shared, the value of an object could be modified in place without having to...

Parsing Email

At the top level, the email module provides two functions for parsing messages message_from_file(f) Parses an email message read from the file-like object f which must be opened in text mode. The input message should be a complete MIME-encoded email message including all headers, text, and attachments. Returns a Message instance. Parses an email message by reading an email message from the text string str. Returns a Message instance. A Message instance m returned by the previous functions...

Pattern Syntax

The following special-character sequences are recognized in regular expression patterns Matches the literal string text. Matches any character except newline. Matches the start of a string. Matches the end of a string. Matches zero or more repetitions of the preceding expression, matching as many repetitions as possible. Matches one or more repetitions of the preceding expression, matching as many repetitions as possible. Matches zero repetitions or one repetition of the preceding expression....

Python it is called fnext

The read() method returns the entire file as a string unless an optional length parameter is given specifying the maximum number of characters. The readline() method returns the next line of input, including the terminating newline the readlines() method returns all the input lines as a list of strings.The readline() method optionally accepts a maximum line length, n. If a line longer than n characters is read, the first n characters are returned. The remaining line data is not discarded and...

Python Services

The following modules provide additional services related to the Python language and execution of the Python interpreter. Many of these modules are related to parsing and compilation of Python source code. Byte-compiles Python files in a directory Register built-in types for use with the pickle module Provides access to the implementation of the import Tests whether a string is a Python keyword Accesses parse trees of Python source code Extracts information for class browsers Compiles Python...

See Also

Fnmatch (p. 316), glob (p. 317), os (p. 378). The signal module is used to write signal handlers in Python. Signals usually correspond to asynchronous events that are sent to a program due to the expiration of a timer, arrival of incoming data, or some action performed by a user. The signal interface emulates that of UNIX, although parts of the module are supported on other platforms. If time is nonzero, a SIGALRM signal is scheduled to be sent to the program in time seconds. Any previously...

Set and Dictionary Comprehensions

The syntax expr for x in s if conditional is a set comprehension. It applies an operation to all of the elements of a set s and can be used in a similar manner as list comprehensions. For example > > > values 1, 2, 3, 4 > > > squares x*x for x in values The syntax kexpr vexpr for k,v in s if condition is a dictionary comprehension. It applies an operation to all of the keys and values in sequence s of (key, value) tuples and returns a dictionary. The keys of the new dictionary are...

Sqlite Module

The sqlite3 module provides a Python interface to the SQLite database library (http www.sqlite.org). SQLite is a C library that implements a self-contained relational database entirely within a file or in memory. Although it is simple, this library is attractive for various reasons. For one, it does not rely upon a separate database server nor does it require any kind of special configuration you can start to use it right away in your programs by simply connecting to a database file (and if it...

Standard Input Output and Error

The interpreter provides three standard file objects, known as standard input, standard output, and standard error, which are available in the sys module as sys.stdin, sys.stdout, and sys.stderr, respectively. stdin is a file object corresponding to the stream of input characters supplied to the interpreter. stdout is the file object that receives output produced by print. stderr is a file that receives error messages. More often than not, stdin is mapped to the user's keyboard, whereas stdout...

String Literals

String literals are used to specify a sequence of characters and are defined by enclosing text in single ('), double (), or triple (''' or ) quotes.There is no semantic difference between quoting styles other than the requirement that you use the same type of quote to start and terminate a string. Single- and double-quoted strings must be defined on a single line, whereas triple-quoted strings can span multiple lines and include all of the enclosed formatting (that is, newlines, tabs, spaces,...

Sum of items with an optional initial value

The + operator concatenates two sequences of the same type. The s * n operator makes n copies of a sequence. However, these are shallow copies that replicate elements by reference only. For example, consider the following code > > > a 3,4,5 > > > b a 3, 4, 5 , 3, 4, 5 , 3, 4, 5 , 3, 4, 5 > > > a 0 -7 -7, 4, 5 , -7, 4, 5 , -7, 4, 5 , -7, 4, 5 Notice how the change to a modified every element of the list c. In this case, a reference to the list a was placed in the list b.When b...

S[ijstrideReturns an extended slice

Checks whether all items in s are True. Checks whether any item in s is True. Operations Applicable to Mutable Sequences Lists support the methods shown in Table 3.4.The built-in function list(s) converts any iterable type to a list. If s is already a list, this function constructs a new list that's a shallow copy of s. The s .append(x) method appends a new element, x, to the end of the list. The s .index(x) method searches the list for the first occurrence of x. If no such element is found, a...

Table List Methods Method

List(s) s.append(x) s.extend(t) s.count(x) Appends a new element, x, to the end of s. Appends a new list, t, to the end of s. Counts occurrences of x in s. ) Returns the smallest i where s i x. start and stop optionally specify the starting and ending index for the search. Returns the element i and removes it from the list. If i is omitted, the last element is returned. Searches for x and removes it from s. Sorts items of s in place. key is a key function. reverse is a flag that sorts the list...

Table of Contents

Variables and Arithmetic Expressions 7 2 Lexical Conventions and Syntax 25 Line Structure and Indentation 25 Identifiers and Reserved Words 26 Numeric Literals 26 String Literals 27 Containers 29 Operators, Delimiters, and Special Symbols 30 Documentation Strings 30 Decorators 30 Source Code Encoding 31 Reference Counting and Garbage Collection 34 First-Class Objects 36 Built-in Types for Representing Data 37 The None Type 38 Numeric Types 38 Sequence Types 39 Mapping Types 44 Set Types 46...

Types and Class Membership Tests

When you create an instance of a class, the type of that instance is the class itself.To test for membership in a class, use the built-in function isinstance(obj, cname) .This function returns True if an object, ojbj,belongs to the class cname or any class derived from cname. Here's an example class A(object) pass class B(A) pass class C(object) pass type(a) Returns the class object A isinstance(b,A) Returns True, B derives from A isinstance(b,C) Returns False, C not derived from A Similarly,...

Using the to Tool

Python 3 includes a tool called 2to3 that can assist with code migration from Python 2.6 to Python 3.This tool is normally found in the Tools scripts directory of the Python source distribution and is also installed in the same directory as the python3.0 binary on most systems. It is a command-line tool that would normally run from a UNIX or Windows command shell. As an example, consider the following program that contains a number of deprecated features. d spam load_spam() return d spam To run...

Variables and Arithmetic Expressions

The program in Listing 1.1 shows the use of variables and expressions by performing a simple compound-interest calculation. Listing 1.1 Simple Compound-Interest Calculation principal 1000 Initial amount print year, principal Reminder print(year, principal) in Python 3 year + 1 The output of this program is the following table Python is a dynamically typed language where variable names are bound to different values, possibly of varying types, during program execution. The assignment operator...

When to Consider Asynchronous Networking

Use of asynchronous I O (asyncore and asynchat), polling, and coroutines as shown in previous examples remains one of the most mysterious aspects of Python develop-ment.Yet, these techniques are used more often than you might think. An often-cited reason for using asynchronous I O is to minimize the perceived overhead of programming with a large number of threads, especially when managing a large number of clients and in light of restrictions related to the global interpreter lock (refer to...

Working with Locks

Great care must be taken when working with any of the locking primitives such as Lock, RLock, or Semaphore. Mismanagement of locks is a frequent source of deadlock or race conditions. Code that relies on a lock should always make sure locks get properly released even when exceptions occur. Typical code looks like this Alternatively, all of the locks also support the context management protocol which is a little cleaner In this last example, the lock is automatically acquired by the with...

Advanced Module Features

The select() and poll() functions are the most generally portable functions defined by this module. On Linux systems, the select module also provides an interface to the edge and level trigger polling (epoll) interface which can offer significantly better performance. On BSD systems, access to kernel queue and event objects is provided.These programming interfaces are described in the online documentation for select at

Using Generators and Coroutines

Here is an example of using these functions to set up a processing pipeline wwwlogs pylines grep(python, lines) In this example, the program is processing all lines in all access-log* files found within all subdirectories of a top-level directory www. Each access-log is tested for file compression and opened using an appropriate file opener. Lines are concatenated together and processed through a filter that is looking for a substring python. The entire program is being driven by the for...

Python Json Reference

The json module is used to serialize and unserialize objects represented using JavaScript Object Notation (JSON). More information about JSON is available at http json.org, but the format is really just a subset of JavaScript syntax. Incidentally, it's almost the same as Python syntax for representing lists and dictionaries. For example, a JSON array is written as value1, value2, , and a JSON object is written as name-.value, name-.value, . The following list shows how JSON values and Python...

Type Conversion from Python to C

The following functions are used by extension modules to convert arguments passed from Python to C. Their prototypes are defined by including the Python.h header file. int PyArg_ParseTuple PyObject args, char format, Parses a tuple of positional arguments in args into a series of C variables. format is a format string containing zero or more of the specifier strings from Tables 26.1 26.3, which describe the expected contents of args. All the remaining arguments contain the addresses of C...

Customization of Application Servers

Other library modules often use the SocketServer class to implement servers for application-level protocols such as HTTP and XML-RPC. Those servers can also be customized via inheritance and extending the methods defined for basic server operation. For example, here is a forking XML-RPC server that only accepts connections originating on the loopback interface from xmlrpc.server import SimpleXMLRPCServer from socketserver import ForkingMixIn except ImportError from SimpleXMLRPCServer import...

List of string classifiers

Creating a setup.py file is enough to create a source distribution of your software. Type the following shell command to make a source distribution This creates an archive file such as spam-1.0.tar.gz or spam-1-0.zip in the directory spam dist.This is the file you would give to others to install your software.To install, a user simply unpacks the archive and performs these steps cd spam-1.0 python setup.py install This installs the software into the local Python distribution and makes it...

Urllib Package

The urllib package provides a high-level interface for writing clients that need to interact with HTTP servers, FTP servers, and local files. Typical applications include scraping data from web pages, automation, proxies, web crawlers, and so forth. This is one of the most highly configurable library modules, so every last detail is not presented here. Instead, the most common uses of the package are described. In Python 2, the urllib functionality is spread across several different library...

XML Example Document

The following example illustrates a typical XML document, in this case a description of a recipe. lt xml version 1.0 encoding iso-8859-1 gt lt recipe gt lt title gt lt item num 4 gt Large avocados, chopped lt item gt lt item num 1 gt Tomato, chopped lt item gt lt item num 1 2 units C gt White onion, chopped lt item gt lt item num 2 units tbl gt Fresh squeezed lemon juice lt item gt lt item num 1 gt Jalapeno pepper, diced lt item gt lt item num 1 units tbl gt Fresh cilantro, minced lt item gt lt...

Http Package

The http package consists of modules for writing HTTP clients and servers as well as support for state management cookies .The Hypertext Transfer Protocol HTTP is a simple text-based protocol that works as follows 1. A client makes a connection to an HTTP server and sends a request header of the following form GET document.html HTTP 1.0 Connection Keep-Alive User-Agent Mozilla 4.61 en X11 U SunOS 5.6 sun4u Host rustler.cs.uchicago.edu 8000 Accept image gif, image x-xbitmap, image jpeg, image...

Symbols Numbers

debugger command, pdb module, 187 not equal to operator, 66 ' single quotes, 11, 27 ''' triple quotes, 11, 27 double quotes, 11, 27 triple quotes, 11, 27 rewriting on package installation, 153 modulo operator, 65 string formatting operator, 8, 70,162 operator, 75 amp bitwise-and operator, 65 amp set intersection operator, 15, 75 amp operator, 75 function call operator, 76 tuple, 14, 29 keyword only arguments, Python 3, 625 passing sequences as function arguments, 94 sequence replication...

Semaphore and Bounded Semaphore

A semaphore is a synchronization primitive based on a counter that's decremented by each acquire call and incremented by each release call. If the counter ever reaches zero, the acquire method blocks until some other thread calls release . Creates a new semaphore. value is the initial value for the counter. If omitted, the counter is set to a value of 1. A Semaphore instance, s, supports the following methods Acquires the semaphore. If the internal counter is larger than zero on entry, this...

Defining Customized Servers

Servers often need special configuration to account for different network address families, timeouts, concurrency, and other features. This customization is carried out by inheriting from one of the four basic servers described in the previous section. The following class attributes can be defined to customize basic settings of the underlying network socket The address family used by the server socket.The default value is socket.AF_INET. Use socket.AF_INET6 if you want to use IPv6. A Boolean...

Predefined Exception Classes

The following exceptions are raised by programs Failed attribute reference or assignment. EOFError End of file. Generated by the built-in functions input and raw_input . It should be noted that most other I O operations such as the read and readline methods of files return an empty string to signal EOF instead of raising an exception. Failed floating-point operation. It should be noted that floating-point exception-handling is a tricky problem and only that this exception only gets raised if...

Pdetach Executes the program and detaches from it The calling program continues to run but cannot wait for the spawned

Spawnv is available on Windows and some versions of UNIX. spawnve mode, path, args, env Executes the program path in a new process, passing the arguments specified in args as command-line parameters and the contents of the mapping env as the environment. args can be a list or a tuple. mode has the same meaning as described for spawnv . The same as spawnv except that all the arguments are supplied as extra parameters. spawnle mode, path, argl, , argn, env The same as spawnve except that the...

Abstract Base Classes

The collections module defines a series of abstract base classes.The purpose of these classes is to describe programming interfaces on various kinds of containers such as lists, sets, and dictionaries. There are two primary uses of these classes. First, they can be used as a base class for user-defined objects that want to emulate the functionality of built-in container types. Second, they can be used for type checking. For example, if you wanted to check that s worked like a sequence, you...

Logging Level Method

CRITICAL log.critical fmt , args , exc_info , extra ERROR log.error fmt , args , exc_info , extra WARNING log.warning fmt , args , exc_info , extra INFO log.info fmt , args , exc_info , extra DEBUG log.debug fmt , args , exc_info , extra The fmt argument is a format string that specifies the format of the log message. Any remaining arguments in args serve as arguments for format specifiers in the format string. The string formatting operator is used to form the resulting message from these...

Managed Objects

Unlike threads, processes do not support shared objects. Although you can create shared values and arrays as shown in the previous section, this doesn't work for more advanced Python objects such as dictionaries, lists, or instances of user-defined classes.The multiprocessing module does, however, provide a way to work with shared objects if they run under the control of a so-called manager.A manager is a separate subprocess where the real objects exist and which operates as a server. Other...

Zip64 Exceed 2gb

If os.path.basename f.name README data t.extractfile f .read print s f.name The tempfile module is used to generate temporary filenames and files. mkdtemp suffix ,prefix , dir Creates a temporary directory accessible only by the owner of the calling process and returns its absolute pathname. suffix is an optional suffix that will be appended to the directory name, prefix is an optional prefix that will be inserted at the beginning of the directory name, and dir is a directory where the...

POSIX Error Codes

The following table shows the POSIX symbolic names for common system error codes. The error codes listed here are supported on almost every version of UNIX, Macintosh OS-X, and Windows. Different UNIX systems may provide additional error codes that are less common and not listed here. If such errors occur, you can consult the errorcode dictionary to find the appropriate symbolic name to use in your program. Address family not supported by protocol. Math argument out of domain of function...

Process Pools

The following class allows you to create a pool of processes to which various kind of data processing tasks can be submitted. The functionality provided by a pool is somewhat similar to that provided by list comprehensions and functional programming operations such as map-reduce. Pool numprocess ,initializer , initargs Creates a pool of worker processes. numprocessis the number of processes to create. If omitted, the value of cpu_count is used. initializer is a callable object that will be...

Web Programming

Ython is widely used when building websites and serves several different roles in this capacity. First, Python scripts are often a useful way to simply generate a set of static HTML pages to be delivered by a web server. For example, a script can be used to take raw content and decorate it with additional features that you typically see on a website navigation bars, sidebars, advertisements, stylesheets, etc. .This is mainly just a matter of file handling and text processing topics that have...