Tutorial :Getting an instance name inside class __init__()



Question:

While building a new class object in python, I want to be able to create a default value based on the instance name of the class without passing in an extra argument. How can I accomplish this? Here's the basic pseudo-code I'm trying for:

class SomeObject():      defined_name = u""        def __init__(self, def_name=None):          if def_name == None:              def_name = u"%s" % (<INSTANCE NAME>)          self.defined_name = def_name    ThisObject = SomeObject()  print ThisObject.defined_name   # Should print "ThisObject"  


Solution:1

Instances don't have names. By the time the global name ThisObject gets bound to the instance created by evaluating the SomeObject constructor, the constructor has finished running.

If you want an object to have a name, just pass the name along in the constructor.

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


Solution:2

Well, there is almost a way to do it:

#!/usr/bin/env python  import traceback  class SomeObject():      def __init__(self, def_name=None):          if def_name == None:              (filename,line_number,function_name,text)=traceback.extract_stack()[-2]              def_name = text[:text.find('=')].strip()          self.defined_name = def_name    ThisObject = SomeObject()  print ThisObject.defined_name   # ThisObject  

The traceback module allows you to peek at the code used to call SomeObject(). With a little string wrangling, text[:text.find('=')].strip() you can guess what the def_name should be.

However, this hack is brittle. For example, this doesn't work so well:

ThisObject,ThatObject = SomeObject(),SomeObject()  print ThisObject.defined_name  # ThisObject,ThatObject  print ThatObject.defined_name   # ThisObject,ThatObject  

So if you were to use this hack, you have to bear in mind that you must call SomeObject() using simple python statement:

ThisObject = SomeObject()  

By the way, as a further example of using traceback, if you define

def pv(var):      # stack is a list of 4-tuples: (filename, line number, function name, text)      # see http://docs.python.org/library/traceback.html#module-traceback      #      (filename,line_number,function_name,text)=traceback.extract_stack()[-2]      # ('x_traceback.py', 18, 'f', 'print_var(y)')      print('%s: %s'%(text[text.find('(')+1:-1],var))  

then you can call

x=3.14  pv(x)  # x: 3.14  

to print both the variable name and its value.


Solution:3

You can create a method inside your class that check all variables in the current frame and use hash() to look for the self variable.

The solution proposed here will return all the variables pointing to the instance object.

In the class below, isinstance() is used to avoid problems when applying hash(), since some objects like a numpy.array or a list, for example, are unhashable.

import inspect  class A(object):      def get_my_name(self):          ans = []          frame = inspect.currentframe().f_back          tmp = dict(frame.f_globals.items() + frame.f_locals.items())          for k, var in tmp.items():              if isinstance(var, self.__class__):                  if hash(self) == hash(var):                      ans.append(k)          return ans  

The following test has been done:

def test():      a = A()      b = a      c = b      print c.get_my_name()  

The result is:

test()  #['a', 'c', 'b']  


Solution:4

This cannot work, just imagine this: a = b = TheMagicObjet(). Names have no effect on Values, they just point to them.


Solution:5

One horrible, horrible way to accomplish this is to reverse the responsibilities:

class SomeObject():      def __init__(self, def_name):          self.defined_name = def_name          globals()[def_name] = self    SomeObject("ThisObject")  print ThisObject.defined_name  

If you wanted to support something other than global scope, you'd have to do something even more awful.


Solution:6

In Python, all data is stored in objects. Additionally, a name can be bound with an object, after which that name can be used to look up that object.

It makes no difference to the object what names, if any, it might be bound to. It might be bound to dozens of different names, or none. Also, Python does not have any "back links" that point from an object to a name.

Consider this example:

foo = 1  bar = foo  baz = foo  

Now, suppose you have the integer object with value 1, and you want to work backwards and find its name. What would you print? Three different names have that object bound to them, and all are equally valid.

print(bar is foo) # prints True  print(baz is foo) # prints True  

In Python, a name is a way to access an object, so there is no way to work with names directly. You could search through various name spaces until you find a name that is bound with the object of interest, but I don't recommend this.

How do I get the string representation of a variable in python?

There is a famous presentation called "Code Like a Pythonista" that summarizes this situation as "Other languages have 'variables'" and "Python has 'names'"

http://python.net/~goodger/projects/pycon/2007/idiomatic/handout.html#other-languages-have-variables


Solution:7

I think that names matters if they are the pointers to any object.. no matters if:

foo = 1  bar = foo  

I know that foo points to 1 and bar points to the same value 1 into the same memory space. but supose that I want to create a class with a function that adds a object to it.

Class Bag(object):     def __init__(self):         some code here...     def addItem(self,item):         self.__dict__[somewaytogetItemName] = item  

So, when I instantiate the class bag like below:

newObj1 = Bag()  newObj2 = Bag()  newObj1.addItem(newObj2)I can do this to get an attribute of newObj1:  newObj1.newObj2  


Solution:8

If you want an unique instance name for a class, try __repr__() or id(self)

class Some:      def __init__(self):          print(self.__repr__())  # = hex(id(self))          print(id(self))  

It will print the memory address of the instance, which is unique.


Solution:9

Inspired by the answers of unutbu and Saullo Castro, I have created a more sophisticated class that can even be subclassed. It solves what was asked for in the question.

"create a default value based on the instance name of the class without passing in an extra argument."

Here's what it does, when an instance of this class or a subclass is created:

  1. Go up in the frame stack until the first frame which does not belong to a method of the current instance.
  2. Inspect this frame to get the attributes self.creation_(name/file/module/function/line/text).
  3. Perform an an additional check whether an object with name self.creation_name was actually defined in the frame's locals() namespace to make 100% sure the found creation_name is correct or raise an error otherwise.

The Code:

import traceback, threading, time    class InstanceCreationError(Exception):      pass    class RememberInstanceCreationInfo:      def __init__(self):          for frame, line in traceback.walk_stack(None):              varnames = frame.f_code.co_varnames              if varnames is ():                  break              if frame.f_locals[varnames[0]] not in (self, self.__class__):                  break                  # if the frame is inside a method of this instance,                  # the first argument usually contains either the instance or                  #  its class                  # we want to find the first frame, where this is not the case          else:              raise InstanceCreationError("No suitable outer frame found.")          self._outer_frame = frame          self.creation_module = frame.f_globals["__name__"]          self.creation_file, self.creation_line, self.creation_function, \              self.creation_text = \              traceback.extract_stack(frame, 1)[0]          self.creation_name = self.creation_text.split("=")[0].strip()          super().__init__()          threading.Thread(target=self._check_existence_after_creation).start()        def _check_existence_after_creation(self):          while self._outer_frame.f_lineno == self.creation_line:              time.sleep(0.01)          # this is executed as soon as the line number changes          # now we can be sure the instance was actually created          error = InstanceCreationError(                  "\nCreation name not found in creation frame.\ncreation_file: "                  "%s \ncreation_line: %s \ncreation_text: %s\ncreation_name ("                  "might be wrong): %s" % (                      self.creation_file, self.creation_line, self.creation_text,                      self.creation_name))          nameparts = self.creation_name.split(".")          try:              var = self._outer_frame.f_locals[nameparts[0]]          except KeyError:              raise error          finally:              del self._outer_frame          # make sure we have no permament inter frame reference          # which could hinder garbage collection          try:              for name in nameparts[1:]: var = getattr(var, name)          except AttributeError:              raise error          if var is not self: raise error        def __repr__(self):          return super().__repr__()[                 :-1] + " with creation_name '%s'>" % self.creation_name  

A simple example:

class MySubclass(RememberInstanceCreationInfo):      def __init__(self):          super().__init__()        def print_creation_info(self):          print(self.creation_name, self.creation_module, self.creation_function,                  self.creation_line, self.creation_text, sep=", ")    instance = MySubclass()  #out: instance, __main__, <module>, 68, instance = MySubclass()  

If the creation name cannot be determined properly an error is raised:

variable, another_instance = 2, MySubclass()    # InstanceCreationError:   # Creation name not found in creation frame.  # creation_file: /.../myfile.py   # creation_line: 71   # creation_text: variable, another_instance = 2, MySubclass()  # creation_name (might be wrong): variable, another_instance  


Solution:10

The best way is really to pass the name to the constructor as in the chosen answer. However, if you REALLY want to avoid asking the user to pass the name to the constructor, you can do the following hack:

If you are creating the instance with 'ThisObject = SomeObject()' from the command line, you can get the object name from the command string in command history:

import readline  import re    class SomeObject():      def __init__(self):          cmd = readline.get_history_item(readline.get_current_history_length())                                                                    self.name = re.split('=| ',cmd)[0]  

If you are creating the instance using 'exec' command, you can handle this with:

if cmd[0:4] == 'exec': self.name = re.split('\'|=| ',cmd)[1]     # if command performed using 'exec'  else: self.name = re.split('=| ',cmd)[0]  

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