Python makes the first one dead simple, the third one maybe unnecessarily hard, and the second one dead simple if you mean lines, but hard otherwise.
How to do each one
Read the whole thing
You can't get much simpler than this:with open(path) as f:
contents = f.read()
dostuff(contents)
Or, if you're dealing with a binary file:with open(path, 'rb') as f:
contents = f.read()
dostuff(contents)
From here on out, I won't distinguish between binary and text files, because the difference is always the same: open in 'rb' mode instead of the default 'r', and you get bytes instead of str.At any rate, either way, you get the entire contents of the file as a single str or bytes in just two lines.
Iterate lines
This is almost as simple:with open(path) as f:
for line in f:
dostuff(line)
Lines don't make as much sense for most binary files—but when they do, it's just as simple; pass the 'rb' mode, and each line is a bytes instead of a str.Iterate chunks
This one is simple for Python experts, but not exactly easy to explain to novices:with open(path, 'rb') as f:
for chunk in iter(lambda: f.read(4096), b''):
dostuff(chunk)
In order to understand this, you have to know the two-argument form of the iter function, how to wrap an expression in a lambda, and that file objects' read method returns an empty bytes (or str, for non-binary files) at EOF. If you didn't know all that, you'd have to write something like this:with open(path, 'rb') as f:
while True:
chunk = f.read(4096)
if not chunk:
break
dostuff(chunk)
Fortunately, you can wrap things up in a function, so at least its uses are easy to understand, even if its implementation isn't:def chunk_file(f, chunksize=4096):
return iter(lambda: f.read(chunksize), b'')
Iterate non-line elements
This one is even trickier. To do the same thing file objects magically do to split on lines, you have to write most of the magic manually: accumulate a buffer between read calls, and split that buffer yourself. Like this:def resplit(buffers, separator):
buf = type(separator)()
for buffer in buffers:
buf += buffer
chunks = buf.split(separator)
yield from chunks[:-1]
buf = chunks[-1]
if buf:
yield buf
with open(path, 'rb') as f:
buffers = chunk_file(f)
for element in resplit(buffers, b'\0'):
dostuff(element)
At least resplit is reusable. But no novice is going to be able to write that. Instead, they'd copy and paste code like this:with open(path, 'rb') as f:
buf = b''
while True:
chunk = f.read(4096)
if not chunk:
break
buf += chunk
elements = buf.split(b'\0')
for element in elements[:-1]:
dostuff(element)
buf = elements[-1]
if buf:
dostuff(buf)
Except, of course, that they'll get all kinds of things wrong, and then have to go back and edit all the copy-pasted copies when they find the bug.Also, notice that most file-like objects (including actual files) already have their own buffer. Throwing another buffer in front of them obviously hurts performance. Less obviously, it also means the file position is ahead of what you've iterated so far (because you've got extra stuff sitting around in your local buf, or the one hidden inside the generator state), so if you wanted to, say, use f.tell() for a progress bar, it wouldn't be accurate.
But what alternative do you have? Obviously you can read one byte at a time; then, you can be sure that each time you've read a line, the file pointer is right at the end of that line. But that's likely to be slow (and especially so with unbuffered raw files).
Binary file objects have a peek method that can help here—instead of read(4096), you just do peek(). If you get anything back, you search for the last separator in the peeked buffer, and, if found, you read that much and split; if not found, you read the length of the peeked buffer and stash it. That's how readline works under the covers on binary files, at least as of CPython 3.4. But that doesn't work for text files, or file-like objects that aren't actual binary files, etc.
Could this be improved?
Novices have to write code like this all the time, and they aren't going to know how to do it. Is there a way Python could make it easier?New file methods
One possibility is to add new methods to file objects. The most important one is a readuntil method (or, alternatively, a new sep parameter for readline, or maybe even a sep parameter for the __init__ method or open function), because you'll need that to get around all of the insurmountable problems with iterating non-line-based elements. But methods like iterchunks and iterlines would also be helpful (and, with some implementations, could be more efficient than what you'd write yourself).For a brand-new language, that would almost certainly be the answer. But for Python? There are thousands of existing file-like classes, many of which do not use the io module to do their job.
If it were easier to wrap file-like objects in io objects, that would be a different story. But it's not. If you've got an object with a read method that returns bytes (what "file-like object" usually means for binary files), that's not something you can wrap in a BufferedReader; you'll need to first create a RawIOBase subclass that delegates to your file-like object and implements readinto, so you can wrap that wrapper a BufferedReader. If you've got an object with a readline method (or __iter__) that returns str (what "file-like object" often means for text files), there's no way at all to wrap a text file with as a TextFileWrapper, except by wrapping it in a BufferedReader that fakes an encoding that you can "decode" cheaply just so you can wrap that up.
Meanwhile, people don't usually want to modify key parts of the standard library until there's significant experience with a third-party module on PyPI. But, at least as of 3.4, there's really no way to write such a module. A readuntil function for binary files is easy (it can call peek if present, go byte by byte if not, just like readline already does), but there's nothing equivalent for text files. (If you're using the _pyio implementation, you can call _get_decoded_chars and _set_decoded_chars to access the internal buffer, but needless to say the builtin C implementation in CPython that you're actually using doesn't expose those internal private methods.) Also, it's not easy to wrap or monkeypatch classes built in C that are returned directly by builtin functions all over the stdlib.
New generic functions
Another option would be to include the chunk_file and split_buffers functions (possibly with better names that I can't think of without more coffee…) in the stdlib, possibly even as builtins. This might not be a bad idea.Files as an iterable of bytes
If you could treat files as iterables of individual characters (or bytes), these functions would become a lot easier to write… but then they'd also become a lot less efficient if it means going back to the file object (or, worse, the actual file) for each character. It might be worth writing an iterbytes()/iterchars() method or helper (but remember that you can already do it with chunk_file(f, 1) or, if you prefer better, chain.from_iterable(chunk_file(f, 4096)), which seems too trivial for the stdlib, and that it'll have all the same problems as the two ideas above…), but it's not a solution.One way to improve on that would be to provide some way for files to act like a sliceable sequence of characters or bytes—or, even better, as a buffer—rather than just an iterable. The mmap module in the stdlib already provides a (not-quite-novice-friendly, but not too bad) way to do this for binary files, but it's not much help for text files, or file-like objects that aren't OS files, or OS files that aren't regular files (like sockets), or files that are too big for your VM space (a problem for anyone on 32-bit systems), etc.
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