Python – How to treat the first line of a file differently in Python

fileiterationpython

I often need to process large text files containing headers in the first line. The headers are often treated differently to the body of the file, or my processing of the body is dependent on the headers. Either way I need to treat the first line as a special case.
I could use simple line iteration and set a flag:

headerProcessed = false
for line in f:
    if headerProcessed:
        processBody(line)
    else:
        processHeader(line)
        headerProcessed = true

but I dislike a test in the loop that is redundant for all but one of the millions of times it executes. Is there a better way? Could I treat the first line differently then get the iteration to start on the second line? Should I be bothered?

Best Answer

You could:

processHeader(f.readline())
for line in f:
    processBody(line)

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Python – How to check whether a file exists without exceptions

If the reason you're checking is so you can do something like if file_exists: open_it(), it's safer to use a try around the attempt to open it. Checking and then opening risks the file being deleted or moved or something between when you check and when you try to open it.

If you're not planning to open the file immediately, you can use os.path.isfile

Return True if path is an existing regular file. This follows symbolic links, so both islink() and isfile() can be true for the same path.

import os.path
os.path.isfile(fname)

if you need to be sure it's a file.

Starting with Python 3.4, the pathlib module offers an object-oriented approach (backported to pathlib2 in Python 2.7):

from pathlib import Path

my_file = Path("/path/to/file")
if my_file.is_file():
    # file exists

To check a directory, do:

if my_file.is_dir():
    # directory exists

To check whether a Path object exists independently of whether is it a file or directory, use exists():

if my_file.exists():
    # path exists

You can also use resolve(strict=True) in a try block:

try:
    my_abs_path = my_file.resolve(strict=True)
except FileNotFoundError:
    # doesn't exist
else:
    # exists

Python – How to execute a program or call a system command

Use the subprocess module in the standard library:

import subprocess
subprocess.run(["ls", "-l"])

The advantage of subprocess.run over os.system is that it is more flexible (you can get the stdout, stderr, the "real" status code, better error handling, etc...).

Even the documentation for os.system recommends using subprocess instead:

The subprocess module provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using this function. See the Replacing Older Functions with the subprocess Module section in the subprocess documentation for some helpful recipes.

On Python 3.4 and earlier, use subprocess.call instead of .run:

subprocess.call(["ls", "-l"])

Best Answer

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Python – How to check whether a file exists without exceptions

Python – How to execute a program or call a system command

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