python-xasm | Python cross version bytecode/wordcode assembler | Bytecode library

PDB tracing uses a trace function set through sys.settrace. There are a number of events that will trigger the trace function, but the ones you're looking at are all line events:

'line'
The interpreter is about to execute a new line of code or re-execute the condition of a loop. The local trace function is called; arg is None; the return value specifies the new local trace function. See Objects/lnotab_notes.txt for a detailed explanation of how this works.

As the docs say, you can see a more detailed explanation of the line event triggers in Objects/lnotab_notes.txt. The most relevant section is

We fix this by only calling the line trace function for a forward jump if the co_lnotab indicates we have jumped to the start of a line, i.e. if the current instruction offset matches the offset given for the start of a line by the co_lnotab. For backward jumps, however, we always call the line trace function, which lets a debugger stop on every evaluation of a loop guard (which usually won't be the first opcode in a line).

So PDB will pause on the start of a line, or if execution jumps backward in the code.

If you want to see the source code that triggers line events, it's in Python/ceval.c under maybe_call_line_trace. PDB's source code is, predictably, under Lib/pdb.py.