[python] open read and close a file in 1 line of code

Now I use:

pageHeadSectionFile = open('pagehead.section.htm','r')
output = pageHeadSectionFile.read()
pageHeadSectionFile.close()

But to make the code look better, I can do:

output = open('pagehead.section.htm','r').read()

When using the above syntax, how do I close the file to free up system resources?

Using CPython, your file will be closed immediately after the line is executed, because the file object is immediately garbage collected. There are two drawbacks, though:

1. In Python implementations different from CPython, the file often isn't immediately closed, but rather at a later time, beyond your control.

2. In Python 3.2 or above, this will throw a ResourceWarning, if enabled.

Better to invest one additional line:

with open('pagehead.section.htm','r') as f:
    output = f.read()

This will ensure that the file is correctly closed under all circumstances.

I think the most natural way for achieving this is to define a function.

def read(filename):
    f = open(filename, 'r')
    output = f.read()
    f.close()
    return output

Then you can do the following:

output = read('pagehead.section.htm')

If you want that warm and fuzzy feeling just go with with.

For python 3.6 I ran these two programs under a fresh start of IDLE, giving runtimes of:

0.002000093460083008  Test A
0.0020003318786621094 Test B: with guaranteed close

So not much of a difference.

#--------*---------*---------*---------*---------*---------*---------*---------*
# Desc: Test A for reading a text file line-by-line into a list
#--------*---------*---------*---------*---------*---------*---------*---------*

import sys
import time

#                                  # MAINLINE
if __name__ == '__main__':
    print("OK, starting program...")

    inTextFile = '/Users/Mike/Desktop/garbage.txt'

#                                  # Test: A: no 'with;
    c=[]
    start_time = time.time()
    c = open(inTextFile).read().splitlines()
    print("--- %s seconds ---" % (time.time() - start_time))

    print("OK, program execution has ended.")
    sys.exit()                     # END MAINLINE

OUTPUT:

OK, starting program...
--- 0.002000093460083008 seconds ---
OK, program execution has ended.

#--------*---------*---------*---------*---------*---------*---------*---------*
# Desc: Test B for reading a text file line-by-line into a list
#--------*---------*---------*---------*---------*---------*---------*---------*

import sys
import time

#                                  # MAINLINE
if __name__ == '__main__':
    print("OK, starting program...")

    inTextFile = '/Users/Mike/Desktop/garbage.txt'

#                                  # Test: B: using 'with'
    c=[]
    start_time = time.time()
    with open(inTextFile) as D: c = D.read().splitlines()
    print("--- %s seconds ---" % (time.time() - start_time))

    print("OK, program execution has ended.")
    sys.exit()                     # END MAINLINE

OUTPUT:

OK, starting program...
--- 0.0020003318786621094 seconds ---
OK, program execution has ended.

Python Standard Library Pathlib module does what you looking for:

Path('pagehead.section.htm').read_text()

Don't forget to import Path:

jsk@dev1:~$ python3
Python 3.5.2 (default, Sep 10 2016, 08:21:44)
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from pathlib import Path
>>> (Path("/etc") / "hostname").read_text()
'dev1.example\n'

On Python 27 install backported pathlib or pathlib2

What you can do is to use the with statement, and write the two steps on one line:

>>> with open('pagehead.section.htm', 'r') as fin: output = fin.read();
>>> print(output)
some content

The with statement will take care to call __exit__ function of the given object even if something bad happened in your code; it's close to the try... finally syntax. For object returned by open, __exit__ corresponds to file closure.

This statement has been introduced with Python 2.6.

use ilio: (inline io):

just one function call instead of file open(), read(), close().

from ilio import read

content = read('filename')

Using more_itertools.with_iter, it is possible to open, read, close and assign an equivalent output in one line (excluding the import statement):

import more_itertools as mit


output = "".join(line for line in mit.with_iter(open("pagehead.section.htm", "r")))

Although possible, I would look for another approach other than assigning the contents of a file to a variable, i.e. lazy iteration - this can be done using a traditional with block or in the example above by removing join() and iterating output.

I frequently do something like this when I need to get a few lines surrounding something I've grepped in a log file:

$ grep -n "xlrd" requirements.txt | awk -F ":" '{print $1}'
54

$ python -c "with open('requirements.txt') as file: print ''.join(file.readlines()[52:55])"
wsgiref==0.1.2
xlrd==0.9.2
xlwt==0.7.5

with open('pagehead.section.htm')as f:contents=f.read()

No need to import any special libraries to do this.

Use normal syntax and it will open the file for reading then close it.

with open("/etc/hostname","r") as f: print f.read() 

or

with open("/etc/hosts","r") as f: x = f.read().splitlines()

which gives you an array x containing the lines, and can be printed like so:

for line in x: print line

These one-liners are very helpful for maintenance - basically self-documenting.