从 python 中的列表中错误地访问值

Question

I have two example files.

myheader.h

#define MACRO1 42
#define lang_init ()  c_init()
#define min(X, Y)  ((X) < (Y) ? (X) : (Y))

and pyparser.py

from pyparsing import *
# define the structure of a macro definition (the empty term is used 
# to advance to the next non-whitespace character)
macroDef = "#define" + Word(alphas+"_",alphanums+"_").setResultsName("macro") + \
            empty + restOfLine.setResultsName("value")
with open('myheader.h', 'r') as f:
    res = macroDef.scanString(f.read())
    res = list(res)
    print(res[0])
    print(res[1])
    print(res[2])

the output is

((['#define', 'MACRO1', '42'], {'macro': ['MACRO1'], 'value': ['42']}), 0, 17) ((['#define', 'lang_init', '() c_init()'], {'macro': ['lang_init'], 'value': ['() c_init()']}), 18, 48) ((['#define', 'min', '(X, Y) ((X) < (Y)? (X): (Y))'], {'macro': ['min'], 'value': ['(X, Y) ((X) < (Y)? (X): (Y))']}), 49, 91)

I thought print(res[0]) would print "#define", print print(res[1]) would print 'MACRO1' and so on. I'm not that familiar with Python, but I'm assuming res is not an array correct? How does indexing works in this case? Thanks

Answer 1

Question, what is the value of len(res).

In python when you have a list inside of a list you can use a second indexer to access the elements inside of it. So for example if the first element res[0] was a list, you could do res[0][0] to get '#define'.

However, your output that you have shown is in a different format than typical nested list syntax, so doing res[0][0] might not work (because it might not be the right type of object).

This is what a nested list is supposed to look like: [[0, 1], [1, 2, 100], [2, 3], [3, 4], [4, 5]]

Your output looks like its in json formatting, but without knowing the type of data object it is for sure I can't be certain. If it is json, you might be able to do json.loads(res) and then parse it that way. https://www.freecodecamp.org/news/python-json-how-to-convert-a-string-to-json/

Answer 2

Part of your confusion stems from a misunderstanding of what you get back from scanString. scanString (which is a legacy compatibility name for the new method scan_string) is actually a generator, which yields a tuple for each found match. The tuple contains:

• the ParseResults returned from matching some text to the scan expression
• the starting position of where the text was found
• the ending position of where the text was found

If you don't want the position values, use search_string instead, which will just return a list of the ParseResults for each match.

Check out the online docs for more details on how to work with scan_string, search_string, ParserElements, and ParseResults. https://pyparsing-docs.readthedocs.io/en/latest/