| Modules | ||||||||||||
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work done on a terminal is lost when the terminal is shut down better to write scripts and save them as files such files are called modules definitions from a module can be imported into other modules fibo.py # Fibonacci numbers module
def fib(n): # write Fibonacci series up to n
a, b = 0, 1
while a < n:
print(a, end=' ')
a, b = b, a+b
print()
def fib2(n): # return Fibonacci series up to n
result = []
a, b = 0, 1
while a < n:
result.append(a)
a, b = b, a+b
return result
through the interpreter
>>> import fibo >>> fibo.fib(1000) 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 >>> fibo.fib2(100) [0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89] >>> fibo.__name__ 'fibo' >>> fib = fibo.fib >>> fib(500) 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 |
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| More on Modules | ||||||||||||
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module can contain executable statements as well as function definitions these statements are intended to initialize the module they are executed only the first time the module name is encountered in an import statement each module has its own private namespace is used as the global namespace by all functions defined in the module modules can import other modules customary but not required to place all import statements at the beginning of a module the imported module names, if placed at the top level of a module, are added to the module's global namespace variant of the import statement that imports names from a module directly into the importing module's namespace >>> from fibo import fib, fib2 >>> fib(500) 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377variant of the import statement that imports all names that a module defines imports all names except those beginning with an underscore >>> from fibo import * >>> fib(500) 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377if the module name is followed by as, then the name following as is bound directly to the imported module >>> import fibo as fib >>> fib.fib(500) 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377can also use aliases >>> from fibo import fib as fibonacci >>> fibonacci(500) 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 |
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| Executing Modules as Scripts | ||||||||||||
run a module as a script using the interpreter
>>> py fibo.py <arguments>code that parses the command line only runs if the module is executed as the 'main' file to permit a module to be both imported and run as a script add to end of module if __name__ == "__main__":
import sys
fib(int(sys.argv[1]))
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| The Module Search Path | ||||||||||||
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when a module named xxx is imported, the interpreter first searches for a built-in module with that name names of built-in modules are listed in sys.builtin_module_names if not found, it then searches for a file named xxx.py in a list of directories given by the variable sys.path sys.path is initialized from
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| "Compiled" Python files | ||||||||||||
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to speed up loading modules, Python caches the compiled version of each module in the __pycache__ directory
under the name module.version.pyc the version encodes the format of the compiled file generally contains the Python version number in CPython release 3.3 the compiled version of xxx.py would be cached as __pycache__/xxx.cpython-33.pyc naming convention allows compiled modules from different releases and different versions of Python to coexist Python checks the modification date of the source against the compiled version to see if it's out of date and needs to be recompiled an automatic process the compiled modules are platform-independent two circumstances when Python does not check the cache
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| Standard Modules | ||||||||||||
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Python comes with a library of standard modules some modules are built into the interpreter these provide access to operations that are not part of the core of the language but are nevertheless built in sys module is built into every Python interpreter the variables sys.ps1 and sys.ps2 define the strings used as primary and secondary prompts >>> import sys
>>> sys.ps1
'>>> '
>>> sys.ps2
'... '
>>> sys.ps1 = 'C> '
C> print('Yuck!')
Yuck!
C>
the variable sys.path is a list of strings that determines the interpreter's search path for modulesis initialized to a default path taken from the environment variable PYTHONPATH if PYTHONPATH is not set path taken from a built-in default can modify it using standard list operations >>> import sys
>>> sys.path.append('/ufs/guido/lib/python')
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| The dir() Function | ||||||||||||
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built-in function dir() is used
to find out which names a module defines returns a sorted list of strings >>> import fibo, sys >>> dir(fibo) ['__name__', 'fib', 'fib2']dir() does not list the names of built-in functions and variables to get a list of those use the standard module builtins >>> import builtins >>> dir(builtins) ['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException', 'BlockingIOError', 'BrokenPipeError', 'BufferError', 'BytesWarning', 'ChildProcessError', 'ConnectionAbortedError', 'ConnectionError', 'ConnectionRefusedError', 'ConnectionResetError', 'DeprecationWarning', 'EOFError', 'Ellipsis', 'EnvironmentError', 'Exception', 'False', 'FileExistsError', 'FileNotFoundError', 'FloatingPointError', 'FutureWarning', 'GeneratorExit', 'IOError', 'ImportError', 'ImportWarning', 'IndentationError', 'IndexError', 'InterruptedError', 'IsADirectoryError', 'KeyError', 'KeyboardInterrupt', 'LookupError', 'MemoryError', 'NameError', 'None', 'NotADirectoryError', 'NotImplemented', 'NotImplementedError', 'OSError', 'OverflowError', 'PendingDeprecationWarning', 'PermissionError', 'ProcessLookupError', 'ReferenceError', 'ResourceWarning', 'RuntimeError', 'RuntimeWarning', 'StopIteration', 'SyntaxError', 'SyntaxWarning', 'SystemError', 'SystemExit', 'TabError', 'TimeoutError', 'True', 'TypeError', 'UnboundLocalError', 'UnicodeDecodeError', 'UnicodeEncodeError', 'UnicodeError', 'UnicodeTranslateError', 'UnicodeWarning', 'UserWarning', 'ValueError', 'Warning', 'ZeroDivisionError', '_', '__build_class__', '__debug__', '__doc__', '__import__', '__name__', '__package__', 'abs', 'all', 'any', 'ascii', 'bin', 'bool', 'bytearray', 'bytes', 'callable', 'chr', 'classmethod', 'compile', 'complex', 'copyright', 'credits', 'delattr', 'dict', 'dir', 'divmod', 'enumerate', 'eval', 'exec', 'exit', 'filter', 'float', 'format', 'frozenset', 'getattr', 'globals', 'hasattr', 'hash', 'help', 'hex', 'id', 'input', 'int', 'isinstance', 'issubclass', 'iter', 'len', 'license', 'list', 'locals', 'map', 'max', 'memoryview', 'min', 'next', 'object', 'oct', 'open', 'ord', 'pow', 'print', 'property', 'quit', 'range', 'repr', 'reversed', 'round', 'set', 'setattr', 'slice', 'sorted', 'staticmethod', 'str', 'sum', 'super', 'tuple', 'type', 'vars', 'zip'] |
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| Packages | ||||||||||||
Packages organize modules in a directory structure into a namespace by using 'dotted module names'
sound/ Top-level package
__init__.py Initialize the sound package
formats/ Subpackage for file format conversions
__init__.py
wavread.py
wavwrite.py
aiffread.py
aiffwrite.py
auread.py
auwrite.py
...
effects/ Subpackage for sound effects
__init__.py
echo.py
surround.py
reverse.py
...
filters/ Subpackage for filters
__init__.py
equalizer.py
vocoder.py
karaoke.py
...
__init__.py files are required to make Python treat directories containing the file as packagesa relatively advanced feature is using a namespace package prevents directories with a common name from unintentionally hiding valid modules in the module search path __init__.py can
loads the submodule sound.effects.echo must be referenced with its full name import sound.effects.echo sound.effects.echo.echofilter(input, output, delay=0.7, atten=4)can also import using aliases from sound.effects import echo echo.echofilter(input, output, delay=0.7, atten=4)can import the desired function or variable directly using an alias from sound.effects.echo import echofilter echofilter(input, output, delay=0.7, atten=4)if an import statement fails an ImportError exception is raised |
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| Importing * From a Package | ||||||||||||
it's not wise to import all submodules from a package
from sound.effects import *doing so could take a long time importing all sub-modules might have side-effects which should only happen when the sub-module is explicitly imported package author can provide an explicit index of the package a package's __init__.py code can define a list of module names that should be imported when from package import * is encountered the __all__ variable is a list of strings that defines the names that are exported when from package import * is used __all__ = ["echo", "surround", "reverse"]sound.effects import * would import the three named submodules of the sound.effects package |
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| Intra-package References | ||||||||||||
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modules within a package can reference each other using relative imports without circular references below moduleA imports moduleC while moduleC imports moduleA
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| Packages in Multiple Directories | ||||||||||||
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packages support one more special attribute __path__ initialized to be a sequence of strings containing the name of the directory holding the package's __init__.py not often needed |