For information - all taken from docs and Lib/*.py

https://docs.python.org/3.7/library/traceback.html
traceback — Print or retrieve a stack traceback
Source code: Lib/traceback.py
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This module provides a standard interface to extract, format and print stack traces of Python programs. It exactly mimics the behavior of the Python interpreter when it prints a stack trace. This is useful when you want to print stack traces under program control, such as in a “wrapper” around the interpreter.
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https://github.com/python/cpython/blob/3.7/Lib/traceback.py#L344-L359
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        for f, lineno in frame_gen:
            co = f.f_code
            filename = co.co_filename
            name = co.co_name

            fnames.add(filename)
            linecache.lazycache(filename, f.f_globals)
            # Must defer line lookups until we have called checkcache.
            if capture_locals:
                f_locals = f.f_locals
            else:
                f_locals = None
            result.append(FrameSummary(
                filename, lineno, name, lookup_line=False, locals=f_locals))
        for filename in fnames:
            linecache.checkcache(filename)
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By the way, here fnames is a set.

https://docs.python.org/3.7/library/linecache.html#module-linecache
linecache — Random access to text lines
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The linecache module allows one to get any line from a Python source file, while attempting to optimize internally, using a cache, the common case where many lines are read from a single file. This is used by the traceback module to retrieve source lines for inclusion in the formatted traceback.
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linecache.checkcache(filename=None)
Check the cache for validity. Use this function if files in the cache may have changed on disk, and you require the updated version. If filename is omitted, it will check all the entries in the cache.

linecache.lazycache(filename, module_globals)
Capture enough detail about a non-file-based module to permit getting its lines later via getline() even if module_globals is None in the later call. This avoids doing I/O until a line is actually needed, without having to carry the module globals around indefinitely.
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