4 ms·

Everything in Python is a reference, and there's no null pointer issues.

auxym 7y ago
I've certainly had some "None" errors in Python. I think the difference comes from dynamic vs static typing. In Python, you sort of get into the habit of "defensive" programming: checking inputs to your function, catching Nones, etc. In java, you tend to rely more on the type system. If it typechecks/compiles, there's a good chance it's OK. That is, until you get a null value that's not handled. That's the root issue I think: If null is an acceptable value per the type, then the same type system should force you to handle it. As do the type systems in ML languages for option types, for example.
- JakobProgsch 7y ago
  The first line is why I'm not 100% convinced of the severity of this mistake compared to the alternatives. The problem fundamentally is the use of magic values/numbers to represent the concept of "no value". You don't need explicit language support to have that concept and the bugs it causes. I guess having that as an intrinsic concept in the language makes it more likely that people use it badly. On the other hand debuggers etc. also intrinsically understand this and segfaults due to null pointers are usually very easy to localize once you see them. On the other hand if a "bad programmer" introduced their own magic non-value in a supposedly safe language, debugging that becomes way more confusing.
  - andrepd 7y ago
    No, that's not the "fundamental problem". The fundamental problem is a type system that lies. A "pointer to string" is not actually a pointer to a string, it's a pointer to a string or to nothing. If your api returns a pointer of the latter type, it should signal this by making the return type "maybe-pointer to string" (although it has the same memory representation as "pointer to string"). Then, if the user tries to dereference a maybe-pointer (that is, to use a maybe-pointer as a pointer), the type system can statically catch this and make it a simple type check failure compilation error. The user must first check if it's null through a function that casts a maybe-pointer to a pointer. Nothing about this precludes the usage of sentinel values.
brudgers 7y ago
Everything in Python is an object. In Python, containers are objects that reference other objects. https://docs.python.org/3/reference/datamodel.html https://docs.python.org/3/reference/datamodel.html https://docs.python.org/2.0/ref/objects.html https://docs.python.org/2.0/ref/objects.html
- dorfsmay 7y ago
  How are name bindings different than references? >>> a=[2, 3, 1] >>> b=a >>> id(a) 139731931982216 >>> id(b) 139731931982216 >>> b [2, 3, 1] >>> b.sort() >>> del(b) >>> a [1, 2, 3] >>>
  - fulafel 7y ago
    Depends what you mean by reference. In Python they are indeed synonymous. C++ references are different in that the binding / object identity can be changed through the reference, eg when you pass a reference to a function.
  - brudgers 7y ago
    In Python, lists are container objects. Container objects reference other objects. In the first line object a references objects 2, 3 ,and 1 (and any other objects in a's object heritage). 2, 3 and 1 have id's. That's what "everything is an object" fleshes out to in Python. But they don't reference other objects because they are literals. The value of 2 is also its name.
kragen 7y ago
If you've never seen this message, you haven't been programming in Python for very long: AttributeError: 'NoneType' object has no attribute 'foo' Not to mention UnboundLocalError and cases of AttributeError that stem from trying to use attributes before they've been initialized. Some of these have slightly better ergonomics than Java’s pernicious null initialization, for example by crashing your program earlier, but the upshot is that everything you do in Java that will crash with a NullPointerException will also still crash your program in Python. Oh, I guess except for shadowing an outer-scope variable with a local that you never initialize. That just gives you the wrong answer in Python, because there exists no local without an initialization. But it's a pretty marginal case.

I've certainly had some "None" errors in Python. I think the difference comes from dynamic vs static typing. In Python, you sort of get into the habit of "defensive" programming: checking inputs to your function, catching Nones, etc. In java, you tend to rely more on the type system. If it typechecks/compiles, there's a good chance it's OK. That is, until you get a null value that's not handled. That's the root issue I think: If null is an acceptable value per the type, then the same type system should force you to handle it. As do the type systems in ML languages for option types, for example.

JakobProgsch 7y ago
The first line is why I'm not 100% convinced of the severity of this mistake compared to the alternatives. The problem fundamentally is the use of magic values/numbers to represent the concept of "no value". You don't need explicit language support to have that concept and the bugs it causes. I guess having that as an intrinsic concept in the language makes it more likely that people use it badly. On the other hand debuggers etc. also intrinsically understand this and segfaults due to null pointers are usually very easy to localize once you see them. On the other hand if a "bad programmer" introduced their own magic non-value in a supposedly safe language, debugging that becomes way more confusing.
- andrepd 7y ago
  No, that's not the "fundamental problem". The fundamental problem is a type system that lies. A "pointer to string" is not actually a pointer to a string, it's a pointer to a string or to nothing. If your api returns a pointer of the latter type, it should signal this by making the return type "maybe-pointer to string" (although it has the same memory representation as "pointer to string"). Then, if the user tries to dereference a maybe-pointer (that is, to use a maybe-pointer as a pointer), the type system can statically catch this and make it a simple type check failure compilation error. The user must first check if it's null through a function that casts a maybe-pointer to a pointer. Nothing about this precludes the usage of sentinel values.

brudgers 7y ago

Everything in Python is an object. In Python, containers are objects that reference other objects. https://docs.python.org/3/reference/datamodel.html https://docs.python.org/3/reference/datamodel.html https://docs.python.org/2.0/ref/objects.html https://docs.python.org/2.0/ref/objects.html

dorfsmay 7y ago
How are name bindings different than references? >>> a=[2, 3, 1] >>> b=a >>> id(a) 139731931982216 >>> id(b) 139731931982216 >>> b [2, 3, 1] >>> b.sort() >>> del(b) >>> a [1, 2, 3] >>>
- fulafel 7y ago
  Depends what you mean by reference. In Python they are indeed synonymous. C++ references are different in that the binding / object identity can be changed through the reference, eg when you pass a reference to a function.
- brudgers 7y ago
  In Python, lists are container objects. Container objects reference other objects. In the first line object a references objects 2, 3 ,and 1 (and any other objects in a's object heritage). 2, 3 and 1 have id's. That's what "everything is an object" fleshes out to in Python. But they don't reference other objects because they are literals. The value of 2 is also its name.

kragen 7y ago

If you've never seen this message, you haven't been programming in Python for very long: AttributeError: 'NoneType' object has no attribute 'foo' Not to mention UnboundLocalError and cases of AttributeError that stem from trying to use attributes before they've been initialized. Some of these have slightly better ergonomics than Java’s pernicious null initialization, for example by crashing your program earlier, but the upshot is that everything you do in Java that will crash with a NullPointerException will also still crash your program in Python. Oh, I guess except for shadowing an outer-scope variable with a local that you never initialize. That just gives you the wrong answer in Python, because there exists no local without an initialization. But it's a pretty marginal case.