We can often consider a file parser to be a kind of reduction. Many languages have two levels of definition: the lower-level tokens in the language and the higher-level structures built from those tokens. When looking at an XML file, the tags, tag names, and attribute names form this lower-level syntax; the structures which are described by XML form a higher-level syntax.
The lower-level lexical scanning is a kind of reduction that takes individual characters and groups them into tokens. This fits well with Python's generator function design pattern. We can often write functions that look as follows:
def lexical_scan(some_source):
for char in some_source:
if some pattern completed: yield token
else: accumulate token
