119 lines
3.5 KiB
Plaintext
119 lines
3.5 KiB
Plaintext
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Accessing attributes on `None` is almost always a logical error and will raise
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an `AttributeError`.
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To fix this issue, make sure not to access an attribute or method on a value
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that can be `None`.
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== Why is this an issue?
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`None` is a built-in object that represents the absence of a value.
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It is often used as a placeholder value for variables that only sometimes hold a
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value or as a return value for method calls that have no result.
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Attributes and methods of symbols that sometimes can be `None` should only be
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accessed in circumstances where it is certain that they are not set to `None`.
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Otherwise, an `AttributeError` is raised, and the program is interrupted.
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Hence, this issue indicates a logical error as it results from incorrect
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assumptions about the state of variables or the results of computations.
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=== What is the potential impact?
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include::../../../shared_content/layc/exception-impact.adoc[]
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If a `None` value can be induced by user input, this issue may even be
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exploited by attackers to disrupt your application or gain information from
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stack traces.
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=== Exceptions
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`None` does support a fixed set of special attributes like `++__class__++` or
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`++__bool__++`, and this issue is not raised when accessing these attributes.
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== How to fix it
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If your code contains `if-else` statements or similar constructs where some
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branches potentially assign the `None` value to a variable, you must ensure
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that this variable is handled safely afterwards.
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I.e., its attributes should not be accessed at all or only after explicitly
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confirming that it is not `None`.
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Similarly, for any function calls that can return `None` under certain
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conditions, carefully confirm that your code avoids these conditions.
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Again, the safest approach is to check for a `None` return value explicitly.
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=== Code examples
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==== Noncompliant code example
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[source,python,diff-id=1,diff-type=noncompliant]
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----
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def render(file_path):
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if os.path.isfile(file_path):
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data = interpret_csv(file_path)
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else:
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data = None
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# ...
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data.plot_graph() # Noncompliant
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----
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==== Compliant solution
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[source,python,diff-id=1,diff-type=compliant]
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----
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def render(file_path):
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if os.path.isfile(file_path):
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data = interpret_csv(file_path)
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else:
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data = None
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# ...
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if data is not None:
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data.plot_graph()
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else:
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print("No data available.")
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----
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=== How does this work?
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In the given example, the function `render` tries to load information into a
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variable `data`, depending on whether `file_path` is a path to a file.
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If this is not the case, `None` is assigned to `data`.
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At the end of the function, a method `plot_graph()` is called on `data`.
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The call is sure to fail if `data` was assigned to `None`.
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This is prevented by checking first whether `data` is not `None` before
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performing the call.
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== Resources
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=== Documentation
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* The Python Data Model on https://docs.python.org/3/reference/datamodel.html#none[None]
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* https://docs.python.org/3/library/exceptions.html#AttributeError[Attribute Error]
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* https://docs.python.org/3/reference/expressions.html#attribute-references[Attribute References]
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=== Articles & blog posts
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* CVE - https://cwe.mitre.org/data/definitions/476[CWE-476: - NULL Pointer Dereference]
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ifdef::env-github,rspecator-view[]
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'''
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== Implementation Specification
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(visible only on this page)
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include::../message.adoc[]
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include::../highlighting.adoc[]
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'''
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== Comments And Links
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(visible only on this page)
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include::../comments-and-links.adoc[]
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endif::env-github,rspecator-view[]
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