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You can compare the returned value of fp.tell() before and after calling the read method. If they return the same value, fp is at eof.

Furthermore, I don't think your example code actually works. The read method to my knowledge never returns None, but it does return an empty string on eof.

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I'd argue that reading from the file is the most reliable way to establish whether it contains more data. It could be a pipe, or another process might be appending data to the file etc.

If you know that's not an issue, you could use something like:

f.tell() == os.fstat(f.fileno()).st_size

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The Python read functions will return an empty string if they reach EOF

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最佳答案

fp.read() reads up to the end of the file, so after it's successfully finished you know the file is at EOF; there's no need to check. If it cannot reach EOF it will raise an exception.

When reading a file in chunks rather than with read(), you know you've hit EOF when read returns less than the number of bytes you requested. In that case, the following read call will return the empty string (not None). The following loop reads a file in chunks; it will call read at most once too many.

assert n > 0
while True:
chunk = fp.read(n)
if chunk == '':
break
process(chunk)

Or, shorter:

for chunk in iter(lambda: fp.read(n), ''):
process(chunk)

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read returns an empty string when EOF is encountered. Docs are here.

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If file is opened in non-block mode, returning less bytes than expected does not mean it's at eof, I'd say @NPE's answer is the most reliable way:

f.tell() == os.fstat(f.fileno()).st_size

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The "for-else" design is often overlooked. See: Python Docs "Control Flow in Loop":

Example

with open('foobar.file', 'rb') as f:
for line in f:
foo()


else:
# No more lines to be read from file
bar()

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When doing binary I/O the following method is useful:

while f.read(1):
f.seek(-1,1)
# whatever

The advantage is that sometimes you are processing a binary stream and do not know in advance how much you will need to read.

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You can use below code snippet to read line by line, till end of file:

line = obj.readline()
while(line != ''):
# Do Something
line = obj.readline()

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f=open(file_name)
for line in f:
print line

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Although I would personally use a with statement to handle opening and closing a file, in the case where you have to read from stdin and need to track an EOF exception, do something like this:

Use a try-catch with EOFError as the exception:

try:
input_lines = ''
for line in sys.stdin.readlines():
input_lines += line
except EOFError as e:
print e

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As python returns empty string on EOF, and not "EOF" itself, you can just check the code for it, written here

f1 = open("sample.txt")


while True:
line = f1.readline()
print line
if ("" == line):
print "file finished"
break;

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I use this function:

# Returns True if End-Of-File is reached
def EOF(f):
current_pos = f.tell()
file_size = os.fstat(f.fileno()).st_size
return current_pos >= file_size

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f = open(filename,'r')
f.seek(-1,2)     # go to the file end.
eof = f.tell()   # get the end of file location
f.seek(0,0)      # go back to file beginning


while(f.tell() != eof):
<body>

You can use the file methods seek() and tell() to determine the position of the end of file. Once the position is found, seek back to the file beginning

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You can use tell() method after reaching EOF by calling readlines() method, like this:

fp=open('file_name','r')
lines=fp.readlines()
eof=fp.tell() # here we store the pointer
# indicating the end of the file in eof
fp.seek(0) # we bring the cursor at the begining of the file
if eof != fp.tell(): # we check if the cursor
do_something()  # reaches the end of the file

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I really don't understand why python still doesn't have such a function. I also don't agree to use the following

f.tell() == os.fstat(f.fileno()).st_size

The main reason is f.tell() doesn't likely to work for some special conditions.

The method works for me is like the following. If you have some pseudocode like the following

while not EOF(f):
line = f.readline()
" do something with line"

You can replace it with:

lines = iter(f.readlines())
while True:
try:
line = next(lines)
" do something with line"
except StopIteration:
break

This method is simple and you don't need to change most of you code.

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Reading a file in batches of BATCH_SIZE lines (the last batch can be shorter):

BATCH_SIZE = 1000  # lines


with open('/path/to/a/file') as fin:
eof = False
while eof is False:
# We use an iterator to check later if it was fully realized. This
# is a way to know if we reached the EOF.
# NOTE: file.tell() can't be used with iterators.
batch_range = iter(range(BATCH_SIZE))
acc = [line for (_, line) in zip(batch_range, fin)]


# DO SOMETHING WITH "acc"


# If we still have something to iterate, we have read the whole
# file.
if any(batch_range):
eof = True

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Get the EOF position of the file:

def get_eof_position(file_handle):
original_position = file_handle.tell()
eof_position = file_handle.seek(0, 2)
file_handle.seek(original_position)
return eof_position

and compare it with the current position: get_eof_position == file_handle.tell().

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Python doesn't have built-in eof detection function but that functionality is available in two ways: f.read(1) will return b'' if there are no more bytes to read. This works for text as well as binary files. The second way is to use f.tell() to see if current seek position is at the end. If you want EOF testing not to change the current file position then you need bit of extra code.

Below are both implementations.

Using tell() method

import os


def is_eof(f):
cur = f.tell()    # save current position
f.seek(0, os.SEEK_END)
end = f.tell()    # find the size of file
f.seek(cur, os.SEEK_SET)
return cur == end

Using read() method

def is_eof(f):
s = f.read(1)
if s != b'':    # restore position
f.seek(-1, os.SEEK_CUR)
return s == b''

How to use this

while not is_eof(my_file):
val = my_file.read(10)

Play with this code.

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Here is a way to do this with the Walrus Operator (new in Python 3.8)

f = open("a.txt", "r")


while (c := f.read(n)):
process(c)


f.close()

Useful Python Docs (3.8):

Walrus operator: https://docs.python.org/3/whatsnew/3.8.html#assignment-expressions

Methods of file objects: https://docs.python.org/3/tutorial/inputoutput.html#methods-of-file-objects

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This code will work for python 3 and above

file=open("filename.txt")
f=file.readlines()   #reads all lines from the file
EOF=-1   #represents end of file
temp=0
for k in range(len(f)-1,-1,-1):
if temp==0:
if f[k]=="\n":
EOF=k
else:
temp+=1
print("Given file has",EOF,"lines")
file.close()

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You can try this code:

import sys
sys.stdin = open('input.txt', 'r') # set std input to 'input.txt'


count_lines = 0
while True:
try:
v = input() # if EOF, it will raise an error
count_lines += 1
except EOFError:
print('EOF', count_lines) # print numbers of lines in file
break

How to find out whether a file is at its `eof`?

Here is a way to do this with the Walrus Operator (new in Python 3.8)

Useful Python Docs (3.8):