1.5 Debugging Builds

Python can be built with several macros to enable extra checks of the interpreter and extension modules. These checks tend to add a large amount of overhead to the runtime so they are not enabled by default.

A full list of the various types of debugging builds is in the file Misc/SpecialBuilds.txt in the Python source distribution. Builds are available that support tracing of reference counts, debugging the memory allocator, or low-level profiling of the main interpreter loop. Only the most frequently-used builds will be described in the remainder of this section.

Compiling the interpreter with the Py_DEBUG macro defined produces what is generally meant by "a debug build" of Python. Py_DEBUG is enabled in the Unix build by adding --with-pydebug to the configure command. It is also implied by the presence of the not-Python-specific _DEBUG macro. When Py_DEBUG is enabled in the Unix build, compiler optimization is disabled.

In addition to the reference count debugging described below, the following extra checks are performed:

• Extra checks are added to the object allocator.
• Extra checks are added to the parser and compiler.
• Downcasts from wide types to narrow types are checked for loss of information.
• A number of assertions are added to the dictionary and set implementations. In addition, the set object acquires a test_c_api method.
• Sanity checks of the input arguments are added to frame creation.
• The storage for long ints is initialized with a known invalid pattern to catch reference to uninitialized digits.
• Low-level tracing and extra exception checking are added to the runtime virtual machine.
• Extra checks are added to the memory arena implementation.
• Extra debugging is added to the thread module.

There may be additional checks not mentioned here.

Defining Py_TRACE_REFS enables reference tracing. When defined, a circular doubly linked list of active objects is maintained by adding two extra fields to every PyObject. Total allocations are tracked as well. Upon exit, all existing references are printed. (In interactive mode this happens after every statement run by the interpreter.) Implied by Py_DEBUG.

Please refer to Misc/SpecialBuilds.txt in the Python source distribution for more detailed information.

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