Python simple parser using tags

Question

First ever code review post, so I hope I'm doing it right. My understanding of this site is that it is a place to post stuff like this for feedback and criticism. If that isn't the case, I apologise for my misunderstanding.

I'm teaching myself Python 3 and I'm pretty near the start. I couldn't find a simple description of a file parser, so I spent a frankly embarrassing amount of time trying to craft one, and this is the result:

def parse(file, startTag, endTag):
    bears=[]
    bearFile = open(file, 'r')
    for bearLine in bearFile:
        if startTag in bearLine:
            bear = bearFile.readline()
            while not (endTag in bear):
                bears.append(bear.strip())
                bear = bearFile.readline()
    return bears

I tested it using:

print(parse("bears.txt", "==start==", "==end=="))

...on a file which looked like this:

This file contains a list of bears.

Bears are below, between the start and end tags.

==start==
Grizzly
Polar
Koala
Panda
Spectacled
Sun
==end==

These are some more tags, listing some false bears:

==start==
Purple
Hairless
Aquatic
Flying
==end==

...and it works! The output is:

['Grizzly', 'Polar', 'Koala', 'Panda', 'Spectacled', 'Sun', 'Purple', 'Hairless', 'Aquatic', 'Flying']

With some obvious flaws, including:

1 - I can't work out how to ignore empty lines between the tags without using another if. I wondered if it might be better to do this later, and strip out all the '' entries, perhaps as an argument option.

2 - It depends on new lines because of the readline method, so using something else like a comma as a delimiter is currently beyond me.

3 - If there is no end tag, it loops forever. try and except are quite new to me right now.

I hope I'm not just wasting everyone's time by being here...

Answer 1

• Python has a style guide, PEP8. I recommend that you follow it, as it makes your code easier to read. In this case it's mostly just using snake_case, rather than camelCase.
• You should use with to automatically close the file. This happens in any circumstance.
• Rather than using if startTag in bearFile, I'd invert the if and continue.
• Rather than using while you can use for still. This, IMO, is easier to understand too.
• You could use a generator function so you don't have to write as much.

Along with a couple of name edits, here's how I'd change your code:

def parse(path, start, end):
    with open(path) as file:
        for line in file:
            if start not in line:
                continue

            for line in file:
                if end in line:
                    break
                yield line.strip()


print(list(parse("bears.txt", "==start==", "==end==")))

---

To address your concerns:

1. Do this out of the function. If you ever do want these empty values, then you'll be duplicating code. You also only need to use:

   [item for item in parse(...) if item]
   

2. You could implement a function, split_delim, that splits on a delimiter. If you make the function simple, then all you'd need to do is add one line to your function:

   def parse(path, start, end):
       with open(path) as file:
           file = split_delim(file, '\n')
           for line in file:
               ...
   

3. My changes should remove this error, due to how Python's for loops work. However I havn't tested this.