Saving and loading objects and using pickle

Question

I  m trying to save and load objects using pickle module   First I declare my objects    gt  gt  gt  class Fruits pass      gt  gt  gt  banana   Fruits     gt  gt  gt  banana color    yellow   gt  gt  gt  banana value   30   After that I open a file called  Fruits obj  previously I created a new  txt file and I renamed  Fruits obj      gt  gt  gt  import pickle  gt  gt  gt  filehandler   open b Fruits obj   wb    gt  gt  gt  pickle dump banana filehandler    After do this I close my session and I began a new one and I put the next  trying to access to the object that it supposed to be saved    file   open  Fruits obj   r   object file   pickle load file    But I have this message   Traceback  most recent call last   File   lt stdin gt    line 1  in  lt module gt  File  C  Python31 lib pickle py   line 1365  in load encoding encoding  errors errors  load   ValueError  read   from the underlying stream did notreturn bytes   I don  t know what to do because I don  t understand this message  Does anyone know How I can load my object  banana   Thank you   EDIT  As some of you have sugested I put    gt  gt  gt  import pickle  gt  gt  gt  file   open  Fruits obj   rb     There were no problem  but the next I put was    gt  gt  gt  object file   pickle load file    And I have error   Traceback  most recent call last   File   lt stdin gt    line 1  in  lt module gt  File  C  Python31 lib pickle py   line 1365  in load encoding encoding  errors errors  load   EOFError

User · Answer

The following works for me   class Fruits  pass  banana   Fruits    banana color    yellow  banana value   30  import pickle  filehandler   open  Fruits obj   wb   pickle dump banana filehandler  filehandler close    file   open  Fruits obj   rb   object file   pickle load file  file close    print object file color  object file value  sep         yellow  30

User · Answer

You didn t open the file in binary mode   open  Fruits obj   rb     Should work   For your second error  the file is most likely empty  which mean you inadvertently emptied it or used the wrong filename or something    This is assuming you really did close your session  If not  then it s because you didn t close the file between the write and the read    I tested your code  and it works

User · Answer

You re forgetting to read it as binary too   In your write part you have   open b Fruits obj   wb     Note the wb part  Write Binary    In the read part you have   file   open  Fruits obj   r     Note the r part  there should be a b too   So replace it with   file   open  Fruits obj   rb     And it will work       As for your second error  it is most likely cause by not closing syncing the file properly   Try this bit of code to write    gt  gt  gt  import pickle  gt  gt  gt  filehandler   open b Fruits obj   wb    gt  gt  gt  pickle dump banana filehandler   gt  gt  gt  filehandler close     And this  unchanged  to read    gt  gt  gt  import pickle  gt  gt  gt  file   open  Fruits obj   rb    gt  gt  gt  object file   pickle load file    A neater version would be using the with statement   For writing    gt  gt  gt  import pickle  gt  gt  gt  with open  Fruits obj    wb   as fp   gt  gt  gt      pickle dump banana  fp    For reading    gt  gt  gt  import pickle  gt  gt  gt  with open  Fruits obj    rb   as fp   gt  gt  gt      banana   pickle load fp

User · Answer

You can use anycache to do the job for you  Assuming you have a function myfunc which creates the instance   from anycache import anycache  class Fruits pass   anycache cachedir   path to your cache       def myfunc       banana   Fruits       banana color    yellow      banana value   30 return banana   Anycache calls myfunc at the first time and pickles the result to a  file in cachedir using an unique identifier  depending on the the function name and the arguments  as filename  On any consecutive run  the pickled object is loaded   If the cachedir is preserved between python runs  the pickled object is taken from the previous python run   The function arguments are also taken into account   A refactored implementation works likewise   from anycache import anycache  class Fruits pass   anycache cachedir   path to your cache       def myfunc color  value      fruit   Fruits       fruit color   color     fruit value   value return fruit

User · Answer

Always open in binary mode  in this case  file   open  Fruits obj   rb

User · Answer

It seems you want to save your class instances across sessions  and using pickle is a decent way to do this   However  there s a package called klepto that abstracts the saving of objects to a dictionary interface  so you can choose to pickle objects and save them to a file  as shown below   or pickle the objects and save them to a database  or instead of use pickle use json  or many other options   The nice thing about klepto is that by abstracting to a common interface  it makes it easy so you don t have to remember the low-level details of how to save via pickling to a file  or otherwise   Note that It works for dynamically added class attributes  which pickle cannot do     dude hilbert gt   python Python 2 7 6  default  Nov 12 2013  13 26 39    GCC 4 2 1 Compatible Apple Clang 4 1   tags Apple clang-421 11 66    on darwin Type  help    copyright    credits  or  license  for more information   gt  gt  gt  from klepto archives import file archive   gt  gt  gt  db   file archive  fruits txt    gt  gt  gt  class Fruits  pass       gt  gt  gt  banana   Fruits    gt  gt  gt  banana color    yellow   gt  gt  gt  banana value   30  gt  gt  gt    gt  gt  gt  db  banana     banana   gt  gt  gt  db dump    gt  gt  gt     Then we restart     dude hilbert gt   python Python 2 7 6  default  Nov 12 2013  13 26 39    GCC 4 2 1 Compatible Apple Clang 4 1   tags Apple clang-421 11 66    on darwin Type  help    copyright    credits  or  license  for more information   gt  gt  gt  from klepto archives import file archive  gt  gt  gt  db   file archive  fruits txt    gt  gt  gt  db load    gt  gt  gt    gt  gt  gt  db  banana   color  yellow   gt  gt  gt     Klepto works on python2 and python3   Get the code here    https   github com uqfoundation

User · Answer

As for your second problem   Traceback  most recent call last    File  quot  lt stdin gt  quot   line 1  in  lt module gt   File  quot C  Python31 lib pickle py quot   line  1365  in load encoding encoding   errors errors  load   EOFError  After you have read the contents of the file  the file pointer will be at the end of the file - there will be no further data to read  You have to rewind the file so that it will be read from the beginning again  file seek 0   What you usually want to do though  is to use a context manager to open the file and read data from it  This way  the file will be automatically closed after the block finishes executing  which will also help you organize your file operations into meaningful chunks  Finally  cPickle is a faster implementation of the pickle module in C  So  In  1   import  pickle as cPickle  In  2   d     quot a quot   1   quot b quot   2   In  4   with open r quot someobject pickle quot    quot wb quot   as output file              cPickle dump d  output file             pickle file will be closed at this point  preventing your from accessing it any further  In  5   with open r quot someobject pickle quot    quot rb quot   as input file              e   cPickle load input file           In  7   print e ------ gt  print e    a   1   b   2

[python] Saving and loading objects and using pickle

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