How to check for end of file in a recursive function?

Question

I would like to implement a recursive function that takes a file as input and processes the lines. The recursion stops when a condition is met or when I am at the end of file.

The problem is I can not find a way to check if I am at the end of the file.

For instance the following code looking for a line meeting condition won't stop if you reach the end of file.

def f_rec(file):
    line=file.readline()
    if not condition(line): # or file end
        return f_rec(file)
    else:
        return line

I am happy with using another data structure than a file object, but it has to be lazily reading the file. I do not want to use a for loop, because I like functional programming. There may be empty lines in my file.

Edit:

To clarify:

This is not a duplicate of any question asking how to check if you are at the end of file. I know very well that you can do this:

for line in file:
   if condition(line):
      return line

What I want to know really is whether or not it there is a way to apply a recursive function to a file, with the file still being read lazily.

Sorry didn't get the question ? I would use a for loop, but as I said, I don't want to do that (not because it is a scholar exercise but because the actual algorithm I want to implement it just much cleaner with recursion) — bkhantun, Apr 18 '19 at 16:35
Just loop over the file object: `for line info file:` don't use recursion. It is particularly inefficient in Python, and you only get a depth of about 1000 before you start asking for stack overflows — juanpa.arrivillaga, Apr 18 '19 at 16:36
@geckos answer is correct. You have to check for an empty string in line to know you have reached the EOF — juanpa.arrivillaga, Apr 18 '19 at 16:39
Read the other answers in the duplicate. And read the answer in this question **which is correct** — juanpa.arrivillaga, Apr 18 '19 at 16:47
To be fair to op, no answers over there are actually recursively reading lines in a file, however inefficient that may be. — Félix Adriyel Gagnon-Grenier, Apr 18 '19 at 19:55

geckos · Accepted Answer · 2019-04-18T16:39:17.403

1

.readline() will return an empty string when it reaches the end of the file, and empty strings are false so this would work

def f_rec(file):
    line=file.readline()
    if line:
        return f_rec(file)
    else:
        return line

edited Apr 18 '19 at 16:39

answered Apr 18 '19 at 16:31

geckos

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Oh sorry I forgot to specify I may have empty lines in my file. Updating the question – bkhantun Apr 18 '19 at 16:32
I see, so there are another options: https://stackoverflow.com/questions/10140281/how-to-find-out-whether-a-file-is-at-its-eof – geckos Apr 18 '19 at 16:37
2

@user160908 the "empty lines" will still have a `'\n'`, so they won't be empty – juanpa.arrivillaga Apr 18 '19 at 16:37
I'm astonished that python has't an `.iseof()` – geckos Apr 18 '19 at 16:38
1

Python has a perfectly reasonable way to simply iterate over the lines of a file, and failing that, and empty string is guaranteed to only be returned from the file object when you are at the EOF – juanpa.arrivillaga Apr 18 '19 at 16:45
funny enough, that does work because indeed empty lines in a file have a '\n', but without @geckos knowing it does :p – bkhantun Apr 18 '19 at 16:51
Yeah! Luck of me! :D – geckos Apr 18 '19 at 19:58

How to check for end of file in a recursive function?

Edit:

1 Answers1