open read and close a file in 1 line of code

Question

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

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with open  pagehead section htm  as f contents f read

User · Answer

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    In Python implementations different from CPython  the file often isn t immediately closed  but rather at a later time  beyond your control  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

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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

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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

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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

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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   gt  gt  gt  from pathlib import Path  gt  gt  gt   Path   etc      hostname   read text    dev1 example n    On Python 27 install backported pathlib or pathlib2

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You don t really have to close it - Python will do it automatically either during garbage collection or at program exit  But as  delnan noted  it s better practice to explicitly close it for various reasons   So  what you can do to keep it short  simple and explicit   with open  pagehead section htm   r   as f      output   f read     Now it s just two lines and pretty readable  I think

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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

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What you can do is to use the with statement  and write the two steps on one line    gt  gt  gt  with open  pagehead section htm    r   as fin  output   fin read     gt  gt  gt  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

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use ilio   inline io    just one function call instead of file open    read    close     from ilio import read  content   read  filename

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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

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

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