How do I look inside a Python object?

问题

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?

回答1:

Python has a strong set of introspection features.

Take a look at the following built-in functions:

type()
dir()
id()
getattr()
hasattr()
globals()
locals()
callable()

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

回答2:

object.__dict__

回答3:

First, read the source.

Second, use the dir() function.

回答4:

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.

回答5:

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.

回答6:

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.

回答7:

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

回答8:

"""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__

回答9:

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.

回答10:

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
)

回答11:

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.

回答12:

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__

回答13:

pprint and dir together work great

回答14:

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

回答15:

Two great tools for inspecting code are:

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

回答16:

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??

回答17:

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

>>>

回答18:

vars(obj) returns the attributes of an object.

回答19:

import pprint

pprint.pprint(obj.__dict__)

pprint.pprint(vars(obj))

回答20:

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

回答21:

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

来源：https://stackoverflow.com/questions/1006169/how-do-i-look-inside-a-python-object

标签

python

introspection