symboldict 0.2.6

A dict class to organize and lazily import symbols

Package documentation

Organize symbols and load them lazily

Install symboldict

python 2:

pip install symboldict

python 3:

pip3 install symboldict

Purpose

The symboldict module implements a dictionary type containing symbolic references to python objects in the hierarchy of importable modules or builtins objects. Examples of symbolic references are 'os.path.isdir', 'telnetlib.Telnet', 'complex.conjugate' etc. References to user-defined objects are equally easy.

The module implements a lazy loading mechanism for these symbols, which means that a dictionary can contain symbols from many different modules without importing any of these modules until this import becomes necessary in user code.

Symboldicts are meant to favor loose coupling between different parts of an application. For example three modules A, B, C may share a common symboldict sy instead of importing each others symbols directly. A and B can use method sy.func() instead of function C.func(), thus decreasing the knowledge that each module possesses about other modules.

Quick Start

SymbolDict objects

A SymbolDict, or dictionary of symbols, is typically created by a statement similar to:

>>> sy = SymbolDict(
...     isfile=symbol.os.path.isfile,
...     Telnet=symbol.telnetlib.Telnet,
...     conju=symbol.complex.conjugate,
...     eggs=symbol.spam.eggs,
... )

It is a regular dictionary, which keys are arbitrary but which values are instances of a new type Symbol:

>>> sy
SymbolDict({'eggs': Symbol('spam.eggs'), 'isfile': Symbol('os.path.isfile'), ...})
>>>
>>> isinstance(sy, dict)
True

The usual dictionary operations apply:

>>> sy['isfile']
Symbol('os.path.isfile')
>>> sy['eggs']
Symbol('spam.eggs')
>>> sy['Parser'] = Symbol('argparse.ArgumentParser')
>>> 'Telnet' in sy
True

Attribute access is overloaded for symboldicts. It supplies a lazy access to the value symbolically represented by the Symbol instance:

>>> sy.isfile
<function isfile at 0x7f7fd0f40050>
>>> sy.Telnet
<class 'telnetlib.Telnet at 0x7fbb3a518db8'>
>>> sy.conju
<method 'conjugate' of 'complex' objects>
>>> sy.eggs
Traceback (most recent call last)
    ...
ImportError: No module named spam

This syntax enables user code to manipulate the SymbolDict instance as if it was a module containing python variables available through qualified names. Actual modules are imported only when it is necessary to do so.

The values of these variables can also be obtained by the following alternative expression, which handles the case where the dictionary keys are not character strings or where they are existing attribute names of the dict class (such as 'update' or 'clear'):

>>> sy().getvalue('isfile')
<function isfile at 0x7f7fd0f40050>
>>> sy().getvalue('Parser')
<class 'argparse.ArgumentParser'>

The call syntax sy() should be stressed here. Calling a SymbolDict instance returns a special lightweight adapter having the type SymbolDictControl. Direct methods cannot be called on symboldicts because the attribute operator is overloaded.

Symbol objects

Symbol objects (used as values in SymbolDict instances) wrap a dot-separated path to a python object, which may exist or not, for example:

>>> a = Symbol('wave.Error')
>>> b = Symbol('complex.conjugate')
>>> c = Symbol('spam.ham.eggs')

Attribute access is overloaded to allow building other instances with the dot syntax:

>>> Symbol('spam').ham.eggs
Symbol('spam.ham.eggs')

A special instance named symbol is defined, which value is Symbol(''). Its path is empty, and it permits expressions such as:

>>> a = symbol.wave.Error
>>> b = symbol.complex.conjugate
>>> c = symbol.spam.ham.eggs

which produce the same result as above.

Defining an instance does not trigger an attempt to retrieve the indicated python object by importing modules or accessing attributes. However, standalone Symbol instances have the ability to fetch this object by calling the getvalue() method:

>>> a().getvalue()
<class 'wave.Error'>

Again, the call syntax a() enables to bypass the overloading of the attribute operator. It returns a special adapter having the type SymbolControl.

A method hasvalue() indicates if a value can be obtained for the symbol’s path. Unlike getvalue(), it does not raise an exception when there is no value:

>>> symbol.spam.ham().hasvalue()
False

This method also exists for symboldicts. It may raise KeyError if the key is missing in the dictionary:

>>> sy().hasvalue('conju')
True

License

This software is licensed under the MIT License

© 2014 Eric Ringeisen