introspection – How do I look inside a Python object?

The Question :

306 people think this question is useful

I’m starting to code in various projects using Python (including Django web development and Panda3D game development).

To help me understand what’s going on, I would like to basically ‘look’ inside the Python objects to see how they tick – like their methods and properties.

So say I have a Python object, what would I need to print out its contents? Is that even possible?

The Question Comments :

The Answer 1

363 people think this answer is useful

Python has a strong set of introspection features.

Take a look at the following built-in functions:

type() and dir() are particularly useful for inspecting the type of an object and its set of attributes, respectively.

The Answer 2

186 people think this answer is useful

object.__dict__

The Answer 3

65 people think this answer is useful

First, read the source.

Second, use the dir() function.

The Answer 4

65 people think this answer is useful

I’m surprised no one’s mentioned help yet!

In [1]: def foo():
   ...:     "foo!"
   ...:

In [2]: help(foo)
Help on function foo in module __main__:

foo()
    foo!

Help lets you read the docstring and get an idea of what attributes a class might have, which is pretty helpful.

The Answer 5

27 people think this answer is useful

If this is for exploration to see what’s going on, I’d recommend looking at IPython. This adds various shortcuts to obtain an objects documentation, properties and even source code. For instance appending a “?” to a function will give the help for the object (effectively a shortcut for “help(obj)”, wheras using two ?’s (“func??“) will display the sourcecode if it is available.

There are also a lot of additional conveniences, like tab completion, pretty printing of results, result history etc. that make it very handy for this sort of exploratory programming.

For more programmatic use of introspection, the basic builtins like dir(), vars(), getattr etc will be useful, but it is well worth your time to check out the inspect module. To fetch the source of a function, use “inspect.getsource” eg, applying it to itself:

>>> print inspect.getsource(inspect.getsource)
def getsource(object):
    """Return the text of the source code for an object.

    The argument may be a module, class, method, function, traceback, frame,
    or code object.  The source code is returned as a single string.  An
    IOError is raised if the source code cannot be retrieved."""
    lines, lnum = getsourcelines(object)
    return string.join(lines, '')

inspect.getargspec is also frequently useful if you’re dealing with wrapping or manipulating functions, as it will give the names and default values of function parameters.

The Answer 6

20 people think this answer is useful

If you’re interested in a GUI for this, take a look at objbrowser. It uses the inspect module from the Python standard library for the object introspection underneath.

objbrowserscreenshot

The Answer 7

9 people think this answer is useful

You can list the attributes of a object with dir() in the shell:

>>> dir(object())
['__class__', '__delattr__', '__doc__', '__format__', '__getattribute__', '__hash__', '__init__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__']

Of course, there is also the inspect module: http://docs.python.org/library/inspect.html#module-inspect

The Answer 8

8 people think this answer is useful
"""Visit http://diveintopython.net/"""

__author__ = "Mark Pilgrim (mark@diveintopython.org)"


def info(object, spacing=10, collapse=1):
    """Print methods and doc strings.

    Takes module, class, list, dictionary, or string."""
    methodList = [e for e in dir(object) if callable(getattr(object, e))]
    processFunc = collapse and (lambda s: " ".join(s.split())) or (lambda s: s)
    print "\n".join(["%s %s" %
                     (method.ljust(spacing),
                      processFunc(str(getattr(object, method).__doc__)))
                     for method in methodList])

if __name__ == "__main__":
    print help.__doc__

The Answer 9

8 people think this answer is useful

Try ppretty

from ppretty import ppretty


class A(object):
    s = 5

    def __init__(self):
        self._p = 8

    @property
    def foo(self):
        return range(10)


print ppretty(A(), indent='    ', depth=2, width=30, seq_length=6,
              show_protected=True, show_private=False, show_static=True,
              show_properties=True, show_address=True)

Output:

__main__.A at 0x1debd68L (
    _p = 8, 
    foo = [0, 1, 2, ..., 7, 8, 9], 
    s = 5
)

The Answer 10

8 people think this answer is useful

While pprint has been mentioned already by others I’d like to add some context.

The pprint module provides a capability to “pretty-print” arbitrary Python data structures in a form which can be used as input to the interpreter. If the formatted structures include objects which are not fundamental Python types, the representation may not be loadable. This may be the case if objects such as files, sockets, classes, or instances are included, as well as many other built-in objects which are not representable as Python constants.

pprint might be in high-demand by developers with a PHP background who are looking for an alternative to var_dump().

Objects with a dict attribute can be dumped nicely using pprint() mixed with vars(), which returns the __dict__ attribute for a module, class, instance, etc.:

from pprint import pprint
pprint(vars(your_object))

So, no need for a loop.

To dump all variables contained in the global or local scope simply use:

pprint(globals())
pprint(locals())

locals() shows variables defined in a function.
It’s also useful to access functions with their corresponding name as a string key, among other usages:

locals()['foo']() # foo()
globals()['foo']() # foo()

Similarly, using dir() to see the contents of a module, or the attributes of an object.

And there is still more.

The Answer 11

7 people think this answer is useful

Others have already mentioned the dir() built-in which sounds like what you’re looking for, but here’s another good tip. Many libraries — including most of the standard library — are distributed in source form. Meaning you can pretty easily read the source code directly. The trick is in finding it; for example:

>>> import string
>>> string.__file__
'/usr/lib/python2.5/string.pyc'

The *.pyc file is compiled, so remove the trailing ‘c’ and open up the uncompiled *.py file in your favorite editor or file viewer:

/usr/lib/python2.5/string.py

I’ve found this incredibly useful for discovering things like which exceptions are raised from a given API. This kind of detail is rarely well-documented in the Python world.

The Answer 12

4 people think this answer is useful

If you want to look at parameters and methods, as others have pointed out you may well use pprint or dir()

If you want to see the actual value of the contents, you can do

object.__dict__

The Answer 13

4 people think this answer is useful

Two great tools for inspecting code are:

  1. IPython. A python terminal that allows you to inspect using tab completion.

  2. Eclipse with the PyDev plugin. It has an excellent debugger that allows you to break at a given spot and inspect objects by browsing all variables as a tree. You can even use the embedded terminal to try code at that spot or type the object and press ‘.’ to have it give code hints for you.

enter image description here

The Answer 14

3 people think this answer is useful

pprint and dir together work great

The Answer 15

3 people think this answer is useful

There is a python code library build just for this purpose: inspect Introduced in Python 2.7

The Answer 16

3 people think this answer is useful

If you are interested to see the source code of the function corresponding to the object myobj, you can type in iPython or Jupyter Notebook:

myobj??

The Answer 17

2 people think this answer is useful
import pprint

pprint.pprint(obj.__dict__)

or

pprint.pprint(vars(obj))

The Answer 18

1 people think this answer is useful

If you want to look inside a live object, then python’s inspect module is a good answer. In general, it works for getting the source code of functions that are defined in a source file somewhere on disk. If you want to get the source of live functions and lambdas that were defined in the interpreter, you can use dill.source.getsource from dill. It also can get the code for from bound or unbound class methods and functions defined in curries… however, you might not be able to compile that code without the enclosing object’s code.

>>> from dill.source import getsource
>>> 
>>> def add(x,y):
...   return x+y
... 
>>> squared = lambda x:x**2
>>> 
>>> print getsource(add)
def add(x,y):
  return x+y

>>> print getsource(squared)
squared = lambda x:x**2

>>> 
>>> class Foo(object):
...   def bar(self, x):
...     return x*x+x
... 
>>> f = Foo()
>>> 
>>> print getsource(f.bar)
def bar(self, x):
    return x*x+x

>>> 

The Answer 19

1 people think this answer is useful

vars(obj) returns the attributes of an object.

The Answer 20

1 people think this answer is useful

Many good tipps already, but the shortest and easiest (not necessarily the best) has yet to be mentioned:

object?

The Answer 21

0 people think this answer is useful

In addition if you want to look inside list and dictionaries, you can use pprint()

The Answer 22

0 people think this answer is useful

In Python 3.8, you can print out the contents of an object by using the __dict__. For example,

class Person():
   pass

person = Person()

## set attributes
person.first = 'Oyinda'
person.last = 'David'

## to see the content of the object
print(person.__dict__)  

{"first": "Oyinda", "last": "David"}

The Answer 23

-4 people think this answer is useful

Try using:

print(object.stringify())

  • where object is the variable name of the object you are trying to inspect.

This prints out a nicely formatted and tabbed output showing all the hierarchy of keys and values in the object.

NOTE: This works in python3. Not sure if it works in earlier versions

UPDATE: This doesn’t work on all types of objects. If you encounter one of those types (like a Request object), use one of the following instead:

  • dir(object())

or

import pprint then: pprint.pprint(object.__dict__)

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