135 lines
4.5 KiB
Plaintext
135 lines
4.5 KiB
Plaintext
================================================================================
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py.code: higher level python code and introspection objects
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================================================================================
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The ``py.code`` part of the pylib contains some functionality to help
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dealing with Python code objects. Even though working with Python's internal
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code objects (as found on frames and callables) can be very powerful, it's
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usually also quite cumbersome, because the API provided by core Python is
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relatively low level and not very accessible.
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The ``py.code`` library tries to simplify accessing the code objects as well
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as creating them. There is a small set of interfaces a user needs to deal with,
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all nicely bundled together, and with a rich set of 'Pythonic' functionality.
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Contents of the library
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=======================
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Every object in the ``py.code`` library wraps a code Python object related
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to code objects, source code, frames and tracebacks: the ``py.code.Code``
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class wraps code objects, ``py.code.Source`` source snippets,
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``py.code.Traceback` exception tracebacks, ``py.code.Frame`` frame
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objects (as found in e.g. tracebacks) and ``py.code.ExceptionInfo`` the
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tuple provided by sys.exc_info() (containing exception and traceback
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information when an exception occurs). Also in the library is a helper function
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``py.code.compile()`` that provides the same functionality as Python's
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built-in 'compile()' function, but returns a wrapped code object.
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The wrappers
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============
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``py.code.Code``
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-------------------
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Code objects are instantiated with a code object or a callable as argument,
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and provide functionality to compare themselves with other Code objects, get to
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the source file or its contents, create new Code objects from scratch, etc.
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A quick example::
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>>> import py
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>>> c = py.code.Code(py.path.local.read)
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>>> c.path.basename
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'common.py'
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>>> isinstance(c.source(), py.code.Source)
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True
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>>> str(c.source()).split('\n')[0]
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"def read(self, mode='r'):"
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``py.code.Source``
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---------------------
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Source objects wrap snippets of Python source code, providing a simple yet
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powerful interface to read, deindent, slice, compare, compile and manipulate
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them, things that are not so easy in core Python.
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Example::
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>>> s = py.code.Source("""\
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... def foo():
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... print "foo"
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... """)
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>>> str(s).startswith('def') # automatic de-indentation!
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True
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>>> s.isparseable()
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True
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>>> sub = s.getstatement(1) # get the statement starting at line 1
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>>> str(sub).strip() # XXX why is the strip() required?!?
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'print "foo"'
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``py.code.Traceback``
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------------------------
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Tracebacks are usually not very easy to examine, you need to access certain
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somewhat hidden attributes of the traceback's items (resulting in expressions
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such as 'fname = tb.tb_next.tb_frame.f_code.co_filename'). The Traceback
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interface (and its TracebackItem children) tries to improve this.
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Example::
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>>> import sys
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>>> try:
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... py.path.local(100) # illegal argument
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... except:
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... exc, e, tb = sys.exc_info()
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>>> t = py.code.Traceback(tb)
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>>> first = t[1] # get the second entry (first is in this doc)
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>>> first.path.basename # second is in py/path/local.py
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'local.py'
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>>> isinstance(first.statement, py.code.Source)
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True
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>>> str(first.statement).strip().startswith('raise ValueError')
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True
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``py.code.Frame``
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--------------------
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Frame wrappers are used in ``py.code.Traceback`` items, and will usually not
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directly be instantiated. They provide some nice methods to evaluate code
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'inside' the frame (using the frame's local variables), get to the underlying
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code (frames have a code attribute that points to a ``py.code.Code`` object)
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and examine the arguments.
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Example (using the 'first' TracebackItem instance created above)::
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>>> frame = first.frame
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>>> isinstance(frame.code, py.code.Code)
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True
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>>> isinstance(frame.eval('self'), py.path.local)
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True
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>>> [namevalue[0] for namevalue in frame.getargs()]
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['cls', 'path']
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``py.code.ExceptionInfo``
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----------------------------
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A wrapper around the tuple returned by sys.exc_info() (will call sys.exc_info()
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itself if the tuple is not provided as an argument), provides some handy
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attributes to easily access the traceback and exception string.
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Example::
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>>> import sys
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>>> try:
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... foobar()
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... except:
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... excinfo = py.code.ExceptionInfo()
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>>> excinfo.typename
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'NameError'
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>>> isinstance(excinfo.traceback, py.code.Traceback)
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True
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>>> excinfo.exconly()
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"NameError: name 'foobar' is not defined"
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