Python introspection

last modified January 29, 2024

In this article we talk about introspection in Python.

Introspection is an act of self examination. In computer programming, introspection is the ability to determine type or properties of objects at runtime. Python programming language has a large support of introspection. Everything in Python is an object. Every object in Python may have attributes and methods. By using introspection, we can dynamically inspect Python objects.

Python dir function

The dir function returns a sorted list of attributes and methods belonging to an object.

>>> dir(()) [’add’, ‘class’, ‘contains’, ‘delattr’, ‘doc’, ‘eq’, ‘format’, ‘ge’, ‘getattribute’, ‘getitem’, ‘getnewargs’, ‘getslice’, ‘gt’, ‘hash’, ‘init’, ‘iter’, ‘le’, ‘len’, ‘lt’, ‘mul’, ‘ne’, ‘new’, ‘reduce’, ‘reduce_ex’, ‘repr’, ‘rmul’, ‘setattr’, ‘sizeof’, ‘str’, ‘subclasshook’, ‘count’, ‘index’]

Here we see an output of the dir function for a tuple object.

>>> print(().doc) tuple() -> empty tuple tuple(iterable) -> tuple initialized from iterable’s items

If the argument is a tuple, the return value is the same object.

Our investigation showed that there is a doc attribute for a tuple object.

direx.py

#!/usr/bin/python

direx.py

import sys

class MyObject:

def init(self): pass

def examine(self): print(self)

o = MyObject()

print(dir(o)) print(dir([])) print(dir({})) print(dir(1)) print(dir()) print(dir(len)) print(dir(sys)) print(dir(“String”))

The example examines several objects using the dir function: a user defined object, native data types, a function, a string, or a number.

Without any argument, dir returns names in the current scope.

>>> dir() [’builtins’, ‘doc’, ‘loader’, ‘name’, ‘package’, ‘spec’] >>> import sys >>>import math, os >>> dir() [’builtins’, ‘doc’, ‘loader’, ‘name’, ‘package’, ‘spec’, ‘math’, ‘sys’]

We execute the dir function before and after we include some modules.

Python type function

The type function returns the type of an object.

typefun.py

#!/usr/bin/python

typefun.py

import sys

def function(): pass

class MyObject:

def init(self): pass

o = MyObject()

print(type(1)) print(type("")) print(type([])) print(type({})) print(type(())) print(type(object)) print(type(function)) print(type(MyObject)) print(type(o)) print(type(sys))

The example prints various types of objects to the console screen.

$ ./typefun.py <class ‘int’> <class ‘str’> <class ’list’> <class ‘dict’> <class ’tuple’> <class ’type’> <class ‘function’> <class ’type’> <class ‘main.MyObject’> <class ‘module’>

The id function

The id returns a special id of an object.

idfun.py

#!/usr/bin/python

idfun.py

import sys

def fun(): pass

class MyObject:

def init(self): pass

o = MyObject()

print(id(1)) print(id("")) print(id({})) print(id([])) print(id(sys)) print(id(fun)) print(id(MyObject)) print(id(o)) print(id(object))

The code example prints ids of various objects, both built-in and custom.

$ ./idfun.py 10914368 139696088742576 139696087935944 139696065155784 139696088325640 139696088244296 21503992 139696087910776 10738720

Python sys module

The sys module provides access to system specific variables and functions used or maintained by the interpreter and to functions that interact strongly with the interpreter. The module allows us to query about the Python environment.

>>> import sys >>> sys.version ‘3.11.1 (tags/v3.11.1:a7a450f, Dec 6 2022, 19:58:39) [MSC v.1934 64 bit (AMD64)]’ >>> sys.platform ‘win32’ >>> sys.path [’’, ‘C:\Users\Jano\AppData\Local\Programs\Python\Python311\python311.zip’, ‘C:\Users\Jano\AppData\Local\Programs\Python\Python311\Lib’, ‘C:\Users\Jano\AppData\Local\Programs\Python\Python311\DLLs’, ‘C:\Users\Jano\AppData\Local\Programs\Python\Python311’, … ‘]

In the above code we examine the Python version, platform, and search path locations.

We can also use the dir function to get a full list of variables and functions of the sys module.

>>> sys.executable ‘C:\Users\Jano\AppData\Local\Programs\Python\Python311\python.exe’ >>> sys.argv [’’] >>> sys.byteorder ’little’

The example presents executable, argv, and byteorder attributes of the sys module.

>>> sys.executable ‘C:\Users\Jano\AppData\Local\Programs\Python\Python311\python.exe’

The executable is a string giving the name of the executable binary for the Python interpreter, on systems where this makes sense.

>>> sys.argv [’’]

This gives a list of command line arguments passed to a Python script.

>>> sys.byteorder ’little’

The byteorder is an indicator of the native byte order. This will have the value ‘big’ on big-endian (most-significant byte first) platforms, and ’little’ on little-endian (least-significant byte first) platforms.

Other introspection

Next we show various other ways of inspecting Python objects.

attrs.py

#!/usr/bin/python

attr.py

def fun(): pass

print(hasattr(object, ‘doc’)) print(hasattr(fun, ‘doc’)) print(hasattr(fun, ‘call’))

print(getattr(object, ‘doc’)) print(getattr(fun, ‘doc’))

The hasattr function checks if an object has an attribute. The getattr function returns the contents of an attribute if there are some.

$ ./attr.py True True True The most base type None

The isinstance function checks if an objects is an instance of a specific class.

>>> print(isinstance.doc) Return whether an object is an instance of a class or of a subclass thereof.

A tuple, as in isinstance(x, (A, B, ...)), may be given as the target to check against. This is equivalent to isinstance(x, A) or isinstance(x, B) or ... etc.

We can get the describtion of a function interactively.

instance.py

#!/usr/bin/python

instance.py

class MyObject:

def init(self): pass

o = MyObject()

print(isinstance(o, MyObject)) print(isinstance(o, object)) print(isinstance(2, int)) print(isinstance(‘str’, str))

As we know, everything is an object in Python; even numbers and strings. The object is a base type of all objects in Python.

$ ./instance.py True True True True

The issubclass function checks if a specific class is a derived class of another class.

subclass.py

#!/usr/bin/python

subclass.py

class Object:

def init(self): pass

class Wall(Object):

def init(self): pass

print(issubclass(Object, Object)) print(issubclass(Object, Wall)) print(issubclass(Wall, Object)) print(issubclass(Wall, Wall))

In our code example, the Wall class is a subclass of the Object class. Object and Wall are also subclasses of themselves. The Object class is not a subclass of class Wall.

$ ./subclass.py True False True True

The doc attribute gives some documentation about an object and the name attribute holds the name of the object.

namedoc.py

#!/usr/bin/python

namedoc.py

def noaction(): ‘‘‘A function, which does nothing’’’ pass

funcs = [noaction, len, str]

for i in funcs:

print(i.name) print(i.doc) print("-" * 75)

In our example, we create a list of three functions: one custom and two native. We go through the list and print the name and the doc attributes.

$ ./namedoc.py noaction A function, which does nothing

len Return the number of items in a container.

str str(object=’’) -> str str(bytes_or_buffer[, encoding[, errors]]) -> str

Create a new string object from the given object. If encoding or errors is specified, then the object must expose a data buffer that will be decoded using the given encoding and error handler. Otherwise, returns the result of object.str() (if defined) or repr(object). encoding defaults to sys.getdefaultencoding(). errors defaults to ‘strict’.

Finally, there is also a callable function. The function checks if an object is a callable object (a function).

callable.py

#!/usr/bin/python

callable.py

class Car:

def setName(self, name):
    self.name = name

def fun(): pass

c = Car()

print(callable(fun)) print(callable(c.setName)) print(callable([])) print(callable(1))

In the code example we check if three objects are callables.

print(callable(fun)) print(callable(c.setName))

The fun function and the setName method are callables. (A method is a function bound to an object.)

$ ./callable.py True True False False

Source

Python inspect module

In this article we have talked about introspection in Python. More tools for doing introspection can be found in the inspect module.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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