Reading lines from a file using python

问题

I have a file with almost 100000 lines. I want to make a cleanning process (lower case, remove stopwords etc) However it takes time.

Example for 10000 the script needs 15 minutes. For all file I expect to take 150 minutes. However it takes 5 hours.

In the start the file use for reading this:

fileinput = open('tweets.txt', 'r')

lines = fileinput.read().lower() #for lower case, however it load all file

for line in fileinput:
    lines = line.lower()

Question: Can I use a way to read the first 10000 lines making the process of cleaning and after that reading the next blog of lines etc?

回答1:

I would highly suggest operating line-by-line instead of reading in the entire file all at once (in other words, don't use .read()).

with open('tweets.txt', 'r') as fileinput:
    for line in fileinput:
        line = line.lower()
        # ... do something with line ...
        # (for example, write the line to a new file, or print it)

This will automatically take advantage of Python's built-in buffering capabilities.

回答2:

Try to work on the file one line at a time:

lowered = []    

with open('tweets.txt', 'r') as handle:
    for line in handle:
        # keep accumulating the results ...
        lowered.append(line.lower())
        # or just dump the to stdout right away
        print(line)

for line in lowered:
    # print or write to file or whatever you require

That way you reduce the memory overhead, which, in case of large files might lead to swapping and kill performance.

Here are some benchmarks on a file with about 1M lines:

# (1) real 0.223    user 0.195  sys 0.026   pcpu 98.71
with open('medium.txt') as handle:
  for line in handle:
      pass

# (2) real 0.295    user 0.262  sys 0.025   pcpu 97.21
with open('medium.txt') as handle:
    for i, line in enumerate(handle):
        pass
    print(i) # 1031124

# (3) real 21.561 user 5.072  sys 3.530   pcpu 39.89
with open('medium.txt') as handle:
    for i, line in enumerate(handle):
        print(line.lower())

# (4) real 1.702  user 1.605  sys 0.089   pcpu 99.50
lowered = []
with open('medium.txt') as handle:
    for i, line in enumerate(handle):
        lowered.append(line.lower())

# (5) real 2.307  user 1.983  sys 0.159   pcpu 92.89
lowered = []
with open('medium.txt', 'r') as handle:
    for i, line in enumerate(handle):
        lowered.append(line.lower())

with open('lowered.txt', 'w') as handle:
    for line in lowered:
        handle.write(line)

You can also iterator over two files at once:

# (6) real 1.944  user 1.666  sys 0.115   pcpu 91.59
with open('medium.txt', 'r') as src, open('lowered.txt', 'w') as sink:
    for i, line in enumerate(src):
        sink.write(line.lower())

Results as table:

# (1) noop                   0.223
# (2) w/ enumerate           0.295
# (4) list buffer            1.702
# (6) on-the-fly             1.944
# (5) r -> list buffer -> w  2.307
# (3) stdout print          21.561

回答3:

Change your script as follows:

with open('tweets.txt', 'r') as fileinput:
  for line in fileinput:
    """do what you need to do with each line"""
    line = line.lower()

So, basically, don't read in the whole file into memory using read(), just iterate over lines of the opened file. When you read a huge file into memory your process may grow to a point where the system needs to swap out parts of it, and that will make it very slow.

来源：https://stackoverflow.com/questions/14154787/reading-lines-from-a-file-using-python

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