When you call the object.__repr__() method in Python you get something like this back:
<__main__.Test object at 0x2aba1c0cf890>
Is there any way to get a hold of the memory address if you overload __repr__(), other then calling super(Class, obj).__repr__() and regexing it out?
The Python manual has this to say about id():
Return the "identity'' of an object.
This is an integer (or long integer)
which is guaranteed to be unique and
constant for this object during its
lifetime. Two objects with
non-overlapping lifetimes may have the
same id() value. (Implementation note:
this is the address of the object.)
So in CPython, this will be the address of the object. No such guarantee for any other Python interpreter, though.
Note that if you're writing a C extension, you have full access to the internals of the Python interpreter, including access to the addresses of objects directly.
You could reimplement the default repr this way:
def __repr__(self):
return '<%s.%s object at %s>' % (
self.__class__.__module__,
self.__class__.__name__,
hex(id(self))
)
Just use
id(object)
There are a few issues here that aren't covered by any of the other answers.
First, id only returns:
the “identity” of an object. This is an integer (or long integer) which is guaranteed to be unique and constant for this object during its lifetime. Two objects with non-overlapping lifetimes may have the same id() value.
In CPython, this happens to be the pointer to the PyObject that represents the object in the interpreter, which is the same thing that object.__repr__ displays. But this is just an implementation detail of CPython, not something that's true of Python in general. Jython doesn't deal in pointers, it deals in Java references (which the JVM of course probably represents as pointers, but you can't see those—and wouldn't want to, because the GC is allowed to move them around). PyPy lets different types have different kinds of id, but the most general is just an index into a table of objects you've called id on, which is obviously not going to be a pointer. I'm not sure about IronPython, but I'd suspect it's more like Jython than like CPython in this regard. So, in most Python implementations, there's no way to get whatever showed up in that repr, and no use if you did.
But what if you only care about CPython? That's a pretty common case, after all.
Well, first, you may notice that id is an integer;* if you want that 0x2aba1c0cf890 string instead of the number 46978822895760, you're going to have to format it yourself. Under the covers, I believe object.__repr__ is ultimately using printf's %p format, which you don't have from Python… but you can always do this:
format(id(spam), '#010x' if sys.maxsize.bit_length() <= 32 else '#18x')
* In 3.x, it's an int. In 2.x, it's an int if that's big enough to hold a pointer—which is may not be because of signed number issues on some platforms—and a long otherwise.
Is there anything you can do with these pointers besides print them out? Sure (again, assuming you only care about CPython).
All of the C API functions take a pointer to a PyObject or a related type. For those related types, you can just call PyFoo_Check to make sure it really is a Foo object, then cast with (PyFoo *)p. So, if you're writing a C extension, the id is exactly what you need.
What if you're writing pure Python code? You can call the exact same functions with pythonapi from ctypes.
Finally, a few of the other answers have brought up ctypes.addressof. That isn't relevant here. This only works for ctypes objects like c_int32 (and maybe a few memory-buffer-like objects, like those provided by numpy). And, even there, it isn't giving you the address of the c_int32 value, it's giving you the address of the C-level int32 that the c_int32 wraps up.
That being said, more often than not, if you really think you need the address of something, you didn't want a native Python object in the first place, you wanted a ctypes object.
Just in response to Torsten, I wasn't able to call addressof() on a regular python object. Furthermore, id(a) != addressof(a). This is in CPython, don't know about anything else.
>>> from ctypes import c_int, addressof
>>> a = 69
>>> addressof(a)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: invalid type
>>> b = c_int(69)
>>> addressof(b)
4300673472
>>> id(b)
4300673392
You can get something suitable for that purpose with:
id(self)
With ctypes, you can achieve the same thing with
>>> import ctypes
>>> a = (1,2,3)
>>> ctypes.addressof(a)
3077760748L
Documentation:
addressof(C instance) -> integer
Return the address of the C instance internal buffer
Note that in CPython, currently id(a) == ctypes.addressof(a), but ctypes.addressof should return the real address for each Python implementation, if
ctypes is supported
memory pointers are a valid notion.
Edit: added information about interpreter-independence of ctypes
I know this is an old question but if you're still programming, in python 3 these days... I have actually found that if it is a string, then there is a really easy way to do this:
>>> spam.upper
<built-in method upper of str object at 0x1042e4830>
>>> spam.upper()
'YO I NEED HELP!'
>>> id(spam)
4365109296
string conversion does not affect location in memory either:
>>> spam = {437 : 'passphrase'}
>>> object.__repr__(spam)
'<dict object at 0x1043313f0>'
>>> str(spam)
"{437: 'passphrase'}"
>>> object.__repr__(spam)
'<dict object at 0x1043313f0>'
You can get the memory address/location of any object by using the 'partition' method of the built-in 'str' type.
Here is an example of using it to get the memory address of an object:
Python 3.8.3 (default, May 27 2020, 02:08:17)
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> object.__repr__(1)
'<int object at 0x7ca70923f0>'
>>> hex(int(object.__repr__(1).partition('object at ')[2].strip('>'), 16))
0x7ca70923f0
>>>
Here, I am using the built-in 'object' class' '__repr__' method with an object/item such as 1 as an argument to return the string and then I am partitioning that string which will return a tuple of the string before the string that I provided, the string that I provided and then the string after the string that I provided, and as the memory location is positioned after 'object at', I can get the memory address as it has partitioned it from that part.
And then as the memory address was returned as the third item in the returned tuple, I can access it with index 2 from the tuple. But then, it has a right angled bracket as a suffix in the string that I obtained, so I use the 'strip' function to remove it, which will return it without the angled bracket. I then transformed the resulted string into an integer with base 16 and then turn it into a hex number.
While it's true that id(object) gets the object's address in the default CPython implementation, this is generally useless... you can't do anything with the address from pure Python code.
The only time you would actually be able to use the address is from a C extension library... in which case it is trivial to get the object's address since Python objects are always passed around as C pointers.
If the __repr__ is overloaded, you may consider __str__ to see the memory address of the variable.
Here is the details of __repr__ versus __str__ by Moshe Zadka in StackOverflow.
There is a way to recovery the value from the 'id' command, here it the TL;DR.
ctypes.cast(memory_address,ctypes.py_object).value
source
Related
I have a url:
https://enterpriseefiling.fcc.gov/dataentry/api/download/dbfile/Current_LMS_Dump.zip
Let
u1 = ['https://enterpriseefiling.fcc.gov/dataentry/api/download/dbfile/Current_LMS_Dump.zip']
I run the following code in my python interpreter
import requests, io
r = requests.get(u1, stream=True)
io.BytesIO(r.content)
I get the following response
<_io.BytesIO object at 0x000002244592F1A8>
My question is: what does this mean? Where is 0x000002244592F1A8? What does 0x000002244592F1A8 refer to?
When python needs to print out an object, and the object doesn't otherwise have a built-in method that tells the interpreter how to print it out (for example, requests.Response and python's built-in list and dict types do have this sort of instruction), python uses this format:
<[objecttype] object at [pointer]>
where pointer is literally a pointer to the object's location in memory. That's what you see here: when you do io.BytesIO(r.content) in your interpreter, you create an io.BytesIO object.
A different method tends to get called when, on the interpreter, you do
>>> print(<object>)
rather than just
>>> <object>
and the io.BytesIO class certainly has methods you can use for more useful output, if you look at its documentation. Try assigning it to a variable instead of printing it:
b = io.BytesIO(r.content)
What does 0x000002244592F1A8 refer to?
It refers to the identity of the object. The number is an implementation detail (in CPython it happens to be the address of the object in memory, the same number returned by the id builtin), but what you can count on is that the number will be different for every BytesIO object currently extant in the process.
That kind of information is included in the __repr__ of many objects because it can come useful when debugging, allowing one to distinguish different objects that might have identical content.
My understanding is that 0x000002244592F1A8 here is the formatted id of the object in memory. I say "formatted", because, if you do id() on the same object, the representation will be a little different (it will be formatted as an int or a long instead of a pointer address:
In [1]: import io
In [2]: obj = io.BytesIO()
In [3]: obj
Out[3]: <_io.BytesIO at 0x10da1ca70>
In [4]: id(obj)
Out[4]: 4523674224
To convert the id() to the format you see, you can do something like this (stolen from this post):
In [5]: format(id(obj), '#010x' if sys.maxsize.bit_length() <= 32 else '#18x')
Out[5]: ' 0x10da1ca70'
This ^^ is not particularly useful, but it just shows you how id() lines up with what you are seeing.
The reason you are seeing it is just that it is displayed as part of the default __repr__() for the BytesIO object.
I'm using SWIG to wrap a piece of C++ code to use it from Python.
Let's say I have an "Array< T >" user-defined class in C++ and I want to compute this line of code from Python:
myArray = MySWIGModule.getObject().myArray
where the method "getObject()" returns a class "Object" containing (amongst other things) an attribute "myArray" of type "Array< float >".
When I want to print the content or type of myArray, I get this:
<Swig Object of type 'Array< float > *
But what I would like to obtain is something like:
<class 'MySWIGModule.FloatArray'>
I tried with many different solutions without success. Do you know how to achieve this result? Thanks!
Write
%template(FloatArray) Array<float>
to have SWIG use that name in the interface. That should be just about perfect with wrapper classes enabled; however, if it’s actually a pointer, SWIG might still use the template name.
Also, be careful of lifetime issues: it’s easy to get a pointer to part of an object that gets destroyed when the Python reference to it is dropped.
I'm trying to test for and fix a bug in pprint++ (edit: the correct link; original link left for posterity) which is coming up because the instancemethod type is not hashable:
In [16]: import pandas as pd
In [17]: type(pd.tslib.NaT).__repr__
Out[17]: <instancemethod __repr__ at 0x1058d2be8>
In [18]: hash(type(pd.tslib.NaT).__repr__)
...
TypeError: unhashable type: 'instancemethod'
But I'm having trouble testing for this issue because I don't know where else I can find an instancemethod in the Python 3 standard library, and I don't want my tests to depend on Pandas.
Specifically, it seems like the "normal" builtin types have "instance methods" that are implemented slightly differently:
In [19]: type(None).__repr__
Out[19]: <slot wrapper '__repr__' of 'NoneType' objects>
In [20]: hash(type(None).__repr__)
Out[20]: -9223372036583849574
So: where can I find an instancemethod in the Python 3 standard library so I can write tests against it? Or is it a special type that doesn't appear there?
(note: this only appears to affect Python 3, as the same method in Python 2 is an unbound method, which is hashable)
This type isn't used in anything that comes with Python, and there's no Python-level API to create objects of this type. However, you can do it with a direct C API call:
import ctypes
PyInstanceMethod_New = ctypes.pythonapi.PyInstanceMethod_New
PyInstanceMethod_New.argtypes = (ctypes.py_object,)
PyInstanceMethod_New.restype = ctypes.py_object
arbitrary_callable = sum
instance_method = PyInstanceMethod_New(arbitrary_callable)
The name instancemethod looks a lot like a bound method object, but it turns out it's something else entirely. It's a weird internal thing that, according to its documentation, is supposed to be the new way for C types to represent their methods, except that the standard C-level API for creating a type doesn't actually use it.
According to conversations on the Python issue tracker, this feature was requested by the developers of Cython and Pyrex. It looks like pandas.tslib.NaT is implemented in Cython, and the Cython implementation actually uses this type, where the standard C API for creating types doesn't.
Note that the situation is completely different on Python 2. On Python 2, this new type didn't exist, and instancemethod was the name of the type of method objects representing ordinary methods written in Python. In Python 3, the new type took that name, and the type of method objects for methods written in Python is now named method.
Python provides it, but basically only as part of their test suite AFAICT (no included batteries use it otherwise). You can make one for testing using the _testcapi module:
>>> import _testcapi
>>> testinstancemethod = _testcapi.instancemethod(str.__repr__)
>>> hash(testinstancemethod)
...
TypeError: unhashable type: 'instancemethod'
I am new to Python and I have been stuck for hours with this problem... I don't know how to convert a variable (type string) to another variable (type instance).
>>from Crypto.PublicKey import RSA
>>from Crypto import Random
>>randomValue = Random.new().read
>>priv = RSA.generate(512, randomValue)
After these lines of code, "priv" is created, and this has type "instance".
And I had to convert this "priv" to type string using str(priv).
>>convertedToStr = str(priv)
>>type(convertedToStr)
<type 'str'>
Now, I need to convert it back to 'instance' and want to get the same thing in value and type as the original "priv". Assume that I cannot use "priv" anymore, and I need to convert "convertedToStr" (type string) into "convertedToStr" (type instance).
Is this ever possible?
Note: The reason I am doing this complex thing is because I have client and server sides and when one side sends a message to the other using sendall(var), it does not allow me to send variable of type 'instance'. So I had to convert it to string before sending it. Now, I want to use that on the receiver side as an variable of type 'instance' but I do not know how to convert it back.
The instance type is used for instances of old-style classes in Python 2. You may want to look at priv.__class__ instead of type(priv) to find out what class it actually has. I expect you'll find that it's class is Crypto.PublicKey.RSA._RSAObject, since that's what the generate function is documented to return.
I don't have the Crypto package installed, so I don't actually know what string you get when you call str on a private key instance. You might be able to parse the string and then call the function Crypto.PublicKey.RSA.construct with appropriate values to reconstruct the key object.
But I think that is doing more work than necessary. Instead of calling str on the key, you should instead call its exportKey method. Then, after you send the string you get back to the other system, you can pass it to Crypto.PublicKey.RSA.importKey.
Note that sending a private key over a network may expose it to eavesdropping, making it useless! You probably shouldn't do it unless the connection between your two systems is encrypted with some other system. Your system is only as secure as its weakest link.
Type instance is nothing specific, you can make a custom class and instantiate it, and it will have type instance:
>>> class x:
... y=1
...
>>> type(x())
<type 'instance'>
You can't arbitrarily convert things to a string by calling str() and guarantee get useful results - it merely asks the object to return a string that could say anything at all. In this case you asked for an RSA private key 512 bytes long and the str() output is ~45 bytes long, there's not 10% of the information needed to get the full object state back from that.
The general problem you're trying to solve is serialization/deserialization, and it's the topic of many modules, libraries and protocols - but luckily RSA keys are easy to convert to useful text and back again (not all objects are).
>>> out = priv.exportKey()
>>> new = RSA.importKey(out)
>>> new == priv
True
NB. when I tried your code, it clearly complained at me that 512 byte keys are weak and refused to generate them, insisting on 1024 bytes or more. You possibly are on an older version, but should specify a longer keylength.
I have started playing with Sage recently, and I've come to suspect that the standard Python int is wrapped in a customized class called Integer in Sage. If I type in type(1) in Python, I get <type 'int'>, however, if I type in the same thing in the sage prompt I get <type 'sage.rings.integer.Integer'>.
If I wanted to replace Python int (or list or dict) with my own custom class, how might it be done? How difficult would it be (e.g. could I do it entirely in Python)?
As an addendum to the other answers: when running any code, Sage has a preprocessing step which converts the Sage-Python to true Python (which is then executed). This is done by the preparse function, e.g.
sage: preparse('a = 1')
'a = Integer(1)'
sage: preparse('2^40')
'Integer(2)**Integer(40)'
sage: preparse('F.<x> = PolynomialRing(ZZ)')
"F = PolynomialRing(ZZ, names=('x',)); (x,) = F._first_ngens(1)"
This step is precisely what allows the transparent use of Integers (in place of ints) and the other non-standard syntax (like the polynomial ring example above and [a..b] etc).
As far as I understand, this is the only way to completely transparently use replacements for the built-in types in Python.
You are able to subclass all of Python's built-in types. For example:
class MyInt(int):
pass
i = MyInt(2)
#i is now an instance of MyInt, but still will behave entirely like an integer.
However, you need to explicitly say each integer is a member of MyInt. So type(1) will still be int, you'll need to do type(MyInt(1)).
Hopefully that's close to what you're looking for.
In the case of Sage, it's easy. Sage has complete control of its own REPL (read-evaluate-print loop), so it can parse the commands you give it and make the parts of your expression into whatever classes it wants. It is not so easy to have standard Python automatically use your integer type for integer literals, however. Simply reassigning the built-in int() to some other type won't do it. You could probably do it with an import filter, that scans each file imported for (say) integer literals and replaces them with MyInt(42) or whatever.