inspect – Inspect live objects¶

Purpose:	The inspect module provides functions for introspecting on live objects and their source code.
Available In:	added in 2.1, with updates in 2.3 and 2.5

The inspect module provides functions for learning about live objects, including modules, classes, instances, functions, and methods. You can use functions in this module to retrieve the original source code for a function, look at the arguments to a method on the stack, and extract the sort of information useful for producing library documentation for your source code. My own CommandLineApp module uses inspect to determine the valid options to a command line program, as well as any arguments and their names so command line programs are self-documenting and the help text is generated automatically.

Module Information¶

The first kind of introspection supported lets you probe live objects to learn about them. For example, it is possible to discover the classes and functions in a module, the methods of a class, etc. Let’s start with the module-level details and work our way down to the function level.

To determine how the interpreter will treat and load a file as a module, use getmoduleinfo(). Pass a filename as the only argument, and the return value is a tuple including the module base name, the suffix of the file, the mode that will be used for reading the file, and the module type as defined in the imp module. It is important to note that the function looks only at the file’s name, and does not actually check if the file exists or try to read the file.

import imp
import inspect
import sys

if len(sys.argv) >= 2:
    filename = sys.argv[1]
else:
    filename = 'example.py'

try:
    (name, suffix, mode, mtype)  = inspect.getmoduleinfo(filename)
except TypeError:
    print 'Could not determine module type of %s' % filename
else:
    mtype_name = { imp.PY_SOURCE:'source',
                   imp.PY_COMPILED:'compiled',
                   }.get(mtype, mtype)

    mode_description = { 'rb':'(read-binary)',
                         'U':'(universal newline)',
                         }.get(mode, '')

    print 'NAME   :', name
    print 'SUFFIX :', suffix
    print 'MODE   :', mode, mode_description
    print 'MTYPE  :', mtype_name

Here are a few sample runs:

$ python inspect_getmoduleinfo.py example.py

NAME   : example
SUFFIX : .py
MODE   : U (universal newline)
MTYPE  : source

$ python inspect_getmoduleinfo.py readme.txt

Could not determine module type of readme.txt

$ python inspect_getmoduleinfo.py notthere.pyc

NAME   : notthere
SUFFIX : .pyc
MODE   : rb (read-binary)
MTYPE  : compiled

Example Module¶

The rest of the examples for this tutorial use a single example file source file, found in PyMOTW/inspect/example.py and which is included below. The file is also available as part of the source distribution associated with this series of articles.

"""Sample file to serve as the basis for inspect examples.
"""

def module_level_function(arg1, arg2='default', *args, **kwargs):
    """This function is declared in the module."""
    local_variable = arg1
    return

class A(object):
    """The A class."""
    def __init__(self, name):
        self.name = name

    def get_name(self):
        "Returns the name of the instance."
        return self.name

instance_of_a = A('sample_instance')

class B(A):
    """This is the B class.
    It is derived from A.
    """

    # This method is not part of A.
    def do_something(self):
        """Does some work"""
        pass

    def get_name(self):
        "Overrides version from A"
        return 'B(' + self.name + ')'

Modules¶

It is possible to probe live objects to determine their components using getmembers(). The arguments to getmembers() are an object to scan (a module, class, or instance) and an optional predicate function that is used to filter the objects returned. The return value is a list of tuples with 2 values: the name of the member, and the type of the member. The inspect module includes several such predicate functions with names like ismodule(), isclass(), etc. You can provide your own predicate function as well.

The types of members that might be returned depend on the type of object scanned. Modules can contain classes and functions; classes can contain methods and attributes; and so on.

import inspect

import example

for name, data in inspect.getmembers(example):
    if name == '__builtins__':
        continue
    print '%s :' % name, repr(data)

This sample prints the members of the example module. Modules have a set of __builtins__, which are ignored in the output for this example because they are not actually part of the module and the list is long.

$ python inspect_getmembers_module.py
A : <class 'example.A'>
B : <class 'example.B'>
__doc__ : 'Sample file to serve as the basis for inspect examples.\n'
__file__ : '/Users/dhellmann/Documents/PyMOTW/branches/inspect/example.pyc'
__name__ : 'example'
instance_of_a : <example.A object at 0xbb810>
module_level_function : <function module_level_function at 0xc8230>

The predicate argument can be used to filter the types of objects returned.

import inspect

import example

for name, data in inspect.getmembers(example, inspect.isclass):
    print '%s :' % name, repr(data)

Notice that only classes are included in the output, now:

$ python inspect_getmembers_module_class.py

A : <class 'example.A'>
B : <class 'example.B'>

Classes¶

Classes are scanned using getmembers() in the same way as modules, though the types of members are different.

import inspect
from pprint import pprint

import example

pprint(inspect.getmembers(example.A))

Since no filtering is applied, the output shows the attributes, methods, slots, and other members of the class:

$ python inspect_getmembers_class.py
[('__class__', <type 'type'>),
 ('__delattr__', <slot wrapper '__delattr__' of 'object' objects>),
 ('__dict__', <dictproxy object at 0xca090>),
 ('__doc__', 'The A class.'),
 ('__getattribute__', <slot wrapper '__getattribute__' of 'object' objects>),
 ('__hash__', <slot wrapper '__hash__' of 'object' objects>),
 ('__init__', <unbound method A.__init__>),
 ('__module__', 'example'),
 ('__new__', <built-in method __new__ of type object at 0x32ff38>),
 ('__reduce__', <method '__reduce__' of 'object' objects>),
 ('__reduce_ex__', <method '__reduce_ex__' of 'object' objects>),
 ('__repr__', <slot wrapper '__repr__' of 'object' objects>),
 ('__setattr__', <slot wrapper '__setattr__' of 'object' objects>),
 ('__str__', <slot wrapper '__str__' of 'object' objects>),
 ('__weakref__', <attribute '__weakref__' of 'A' objects>),
 ('get_name', <unbound method A.get_name>)]

To find the methods of a class, use the ismethod() predicate:

import inspect
from pprint import pprint

import example

pprint(inspect.getmembers(example.A, inspect.ismethod))

$ python inspect_getmembers_class_methods.py

[('__init__', <unbound method A.__init__>),
 ('get_name', <unbound method A.get_name>)]

If we look at class B, we see the over-ride for get_name() as well as the new method, and the inherited __init__() method implented in A.

import inspect
from pprint import pprint

import example

pprint(inspect.getmembers(example.B, inspect.ismethod))

Notice that even though __init__() is inherited from A, it is identified as a method of B.

$ python inspect_getmembers_class_methods_b.py

[('__init__', <unbound method B.__init__>),
 ('do_something', <unbound method B.do_something>),
 ('get_name', <unbound method B.get_name>)]

Documentation Strings¶

The docstring for an object can be retrieved with getdoc(). The return value is the __doc__ attribute with tabs expanded to spaces and with indentation made uniform.

import inspect
import example

print 'B.__doc__:'
print example.B.__doc__
print
print 'getdoc(B):'
print inspect.getdoc(example.B)

Notice the difference in indentation on the second line of the doctring:

$ python inspect_getdoc.py

B.__doc__:
This is the B class.
    It is derived from A.


getdoc(B):
This is the B class.
It is derived from A.

In addition to the actual docstring, it is possible to retrieve the comments from the source file where an object is implemented, if the source is available. The getcomments() function looks at the source of the object and finds comments on lines preceding the implementation.

import inspect
import example

print inspect.getcomments(example.B.do_something)

The lines returned include the comment prefix, but any whitespace prefix is stripped off.

$ python inspect_getcomments_method.py

# This method is not part of A.

When a module is passed to getcomments(), the return value is always the first comment in the module.

import inspect
import example

print inspect.getcomments(example)

Notice that contiguous lines from the example file are included as a single comment, but as soon as a blank line appears the comment is stopped.

$ python inspect_getcomments_module.py

# This comment appears first
# and spans 2 lines.

Retrieving Source¶

If the .py file is available for a module, the original source code for the class or method can be retrieved using getsource() and getsourcelines().

import inspect
import example

print inspect.getsource(example.A.get_name)

The original indent level is retained in this case.

$ python inspect_getsource_method.py

    def get_name(self):
        "Returns the name of the instance."
        return self.name

When a class is passed in, all of the methods for the class are included in the output.

import inspect
import example

print inspect.getsource(example.A)

$ python inspect_getsource_class.py

class A(object):
    """The A class."""
    def __init__(self, name):
        self.name = name

    def get_name(self):
        "Returns the name of the instance."
        return self.name

If you need the lines of source split up, it can be easier to use getsourcelines() instead of getsource(). The return value from getsourcelines() is a tuple containing a list of strings (the lines from the source file), and a starting line number in the file where the source appears.

import inspect
import pprint
import example

pprint.pprint(inspect.getsourcelines(example.A.get_name))

$ python inspect_getsourcelines_method.py

(['    def get_name(self):\n',
  '        "Returns the name of the instance."\n',
  '        return self.name\n'],
 48)

If the source file is not available, getsource() and getsourcelines() raise an IOError.

Method and Function Arguments¶

In addition to the documentation for a function or method, it is possible to ask for a complete specification of the arguments the callable takes, including default values. The getargspec() function returns a tuple containing the list of positional argument names, the name of any variable positional arguments (e.g., *args), the names of any variable named arguments (e.g., **kwds), and default values for the arguments. If there are default values, they match up with the end of the positional argument list.

import inspect
import example

arg_spec = inspect.getargspec(example.module_level_function)
print 'NAMES   :', arg_spec[0]
print '*       :', arg_spec[1]
print '**      :', arg_spec[2]
print 'defaults:', arg_spec[3]

args_with_defaults = arg_spec[0][-len(arg_spec[3]):]
print 'args & defaults:', zip(args_with_defaults, arg_spec[3])

Note that the first argument, arg1, does not have a default value. The single default therefore is matched up with arg2.

$ python inspect_getargspec_function.py

NAMES   : ['arg1', 'arg2']
*       : args
**      : kwargs
defaults: ('default',)
args & defaults: [('arg2', 'default')]

Class Hierarchies¶

inspect includes two methods for working directly with class hierarchies. The first, getclasstree(), creates a tree-like data structure using nested lists and tuples based on the classes it is given and their base classes. Each element in the list returned is either a tuple with a class and its base classes, or another list containing tuples for subclasses.

import inspect
import example

class C(example.B):
    pass

class D(C, example.A):
    pass

def print_class_tree(tree, indent=-1):
    if isinstance(tree, list):
        for node in tree:
            print_class_tree(node, indent+1)
    else:
        print '  ' * indent, tree[0].__name__
    return

if __name__ == '__main__':
    print 'A, B, C, D:'
    print_class_tree(inspect.getclasstree([example.A, example.B, C, D]))

The output from this example is the “tree” of inheritance for the A, B, C, and D classes. Note that D appears twice, since it inherits from both C and A.

$ python inspect_getclasstree.py

A, B, C, D:
 object
   A
     D
     B
       C
         D

If we call getclasstree() with unique=True, the output is different.

import inspect
import example
from inspect_getclasstree import *

print_class_tree(inspect.getclasstree([example.A, example.B, C, D],
                                      unique=True,
                                      ))

This time, D only appears in the output once:

$ python inspect_getclasstree_unique.py

 object
   A
     B
       C
         D

Method Resolution Order¶

The other function for working with class hierarchies is getmro(), which returns a tuple of classes in the order they should be scanned when resolving an attribute that might be inherited from a base class. Each class in the sequence appears only once.

import inspect
import example

class C(object):
    pass

class C_First(C, example.B):
    pass

class B_First(example.B, C):
    pass

print 'B_First:'
for c in inspect.getmro(B_First):
    print '\t', c.__name__
print
print 'C_First:'
for c in inspect.getmro(C_First):
    print '\t', c.__name__

This output demonstrates the “depth-first” nature of the MRO search. For B_First, A also comes before C in the search order, because B is derived from A.

$ python inspect_getmro.py

B_First:
        B_First
        B
        A
        C
        object

C_First:
        C_First
        C
        B
        A
        object

The Stack and Frames¶

In addition to introspection of code objects, inspect includes functions for inspecting the runtime environment while a program is running. Most of these functions work with the call stack, and operate on “call frames”. Each frame record in the stack is a 6 element tuple containing the frame object, the filename where the code exists, the line number in that file for the current line being run, the function name being called, a list of lines of context from the source file, and the index into that list of the current line. Typically such information is used to build tracebacks when exceptions are raised. It can also be useful when debugging programs, since the stack frames can be interrogated to discover the argument values passed into the functions.

currentframe() returns the frame at the top of the stack (for the current function). getargvalues() returns a tuple with argument names, the names of the variable arguments, and a dictionary with local values from the frame. By combining them, we can see the arguments to functions and local variables at different points in the call stack.

import inspect

def recurse(limit):
    local_variable = '.' * limit
    print limit, inspect.getargvalues(inspect.currentframe())
    if limit <= 0:
        return
    recurse(limit - 1)
    return

if __name__ == '__main__':
    recurse(3)

The value for local_variable is included in the frame’s local variables even though it is not an argument to the function.

$ python inspect_getargvalues.py

ArgInfo(args=['limit'], varargs=None, keywords=None, locals={'local_variable': '...', 'limit': 3})
ArgInfo(args=['limit'], varargs=None, keywords=None, locals={'local_variable': '..', 'limit': 2})
ArgInfo(args=['limit'], varargs=None, keywords=None, locals={'local_variable': '.', 'limit': 1})
ArgInfo(args=['limit'], varargs=None, keywords=None, locals={'local_variable': '', 'limit': 0})

Using stack(), it is also possible to access all of the stack frames from the current frame to the first caller. This example is similar to the one above, except it waits until reaching the end of the recursion to print the stack information.

import inspect

def recurse(limit):
    local_variable = '.' * limit
    if limit <= 0:
        for frame, filename, line_num, func, source_code, source_index in inspect.stack():
            print '%s[%d]\n  -> %s' % (filename, line_num, source_code[source_index].strip())
            print inspect.getargvalues(frame)
            print
        return
    recurse(limit - 1)
    return

if __name__ == '__main__':
    recurse(3)

The last part of the output represents the main program, outside of the recurse function.

$ python inspect_stack.py
inspect_stack.py[37]
  -> for frame, filename, line_num, func, source_code, source_index in inspect.stack():
(['limit'], None, None, {'local_variable': '', 'line_num': 37, 'frame': <frame object at 0x61ba30>,
'filename': 'inspect_stack.py', 'limit': 0, 'func': 'recurse', 'source_index': 0,
'source_code': ['        for frame, filename, line_num, func, source_code, source_index in inspect.stack():\n']})

inspect_stack.py[42]
  -> recurse(limit - 1)
(['limit'], None, None, {'local_variable': '.', 'limit': 1})

inspect_stack.py[42]
  -> recurse(limit - 1)
(['limit'], None, None, {'local_variable': '..', 'limit': 2})

inspect_stack.py[42]
  -> recurse(limit - 1)
(['limit'], None, None, {'local_variable': '...', 'limit': 3})

inspect_stack.py[46]
  -> recurse(3)
([], None, None, {'__builtins__': <module '__builtin__' (built-in)>,
'__file__': 'inspect_stack.py',
'inspect': <module 'inspect' from '/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/inspect.pyc'>,
'recurse': <function recurse at 0xc81b0>, '__name__': '__main__',
'__doc__': 'Inspecting the call stack.\n\n'})

There are other functions for building lists of frames in different contexts, such as when an exception is being processed. See the documentation for trace(), getouterframes(), and getinnerframes() for more details.

PyMOTW

Page Contents

Navigation

This Page

Examples