How to append data to a json file

Question

I m trying to create a function that would add entries to a json file  Eventually  I want a file that looks like      name     name1    url     url1      name     name2    url     url2      etc  This is what I have   def add args       with open DATA FILENAME  mode  r   encoding  utf-8   as feedsjson          feeds   json load feedsjson      with open DATA FILENAME  mode  w   encoding  utf-8   as feedsjson          entry              entry  name     args name         entry  url     args url         json dump entry  feedsjson    This does create an entry such as   name   some name    url   some url    But  if I use this add function again  with different name and url  the first one gets overwritten  What do I need to do to get a second  third     entry appended to the first one   EDIT  The first answers and comments to this question have pointed out the obvious fact that I am not using feeds in the write block  I don t see how to do that  though  For example  the following apparently will not do   with open DATA FILENAME  mode  a    encoding  utf-8   as feedsjson      feeds   json load feedsjson      entry          entry  name     args name     entry  url     args url     json dump entry  feeds

User · Answer

this  work for me    with open  file json    a   as outfile      outfile write json dumps data       outfile write          outfile close

User · Answer

json might not be the best choice for on-disk formats   The trouble it has with appending data is a good example of why this might be   Specifically  json objects have a syntax that means the whole object must be read and parsed in order to understand any part of it   Fortunately  there are lots of other options   A particularly simple one is CSV  which is supported well by python s standard library   The biggest downside is that it only works well for text  it requires additional action on the part of the programmer to convert the values to numbers or other formats  if needed   Another option which does not have this limitation is to use a sqlite database  which also has built-in support in python   This would probably be a bigger departure from the code you already have  but it more naturally supports the  modify a little bit  model you are apparently trying to build

User · Answer

You can simply import the data from the source file  read it  and save what you want to append to a variable  Then open the destination file  assign the list data inside to a new variable  presumably this will all be valid JSON   then use the  append  function on this list variable and append the first variable to it  Viola  you have appended to the JSON list  Now just overwrite your destination file with the newly appended list  as JSON   The  a  mode in your  open  function will not work here because it will just tack everything on to the end of the file  which will make it non-valid JSON format

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You probably want to use a JSON list instead of a dictionary as the toplevel element   So  initialize the file with an empty list   with open DATA FILENAME  mode  w   encoding  utf-8   as f      json dump     f    Then  you can append new entries to this list   with open DATA FILENAME  mode  w   encoding  utf-8   as feedsjson      entry     name   args name   url   args url      feeds append entry      json dump feeds  feedsjson    Note that this will be slow to execute because you will rewrite the full contents of the file every time you call add  If you are calling it in a loop  consider adding all the feeds to a list in advance  then writing the list out in one go

User · Answer

I have some code which is similar  but does not rewrite the entire contents each time  This is meant to run periodically and append a JSON entry at the end of an array   If the file doesn t exist yet  it creates it and dumps the JSON into an array  If the file has already been created  it goes to the end  replaces the   with a   drops the new JSON object in  and then closes it up again with another      Append JSON object to output file JSON array fname    somefile txt  if os path isfile fname         File exists     with open fname   a    as outfile          outfile seek -1  os SEEK END          outfile truncate           outfile write              json dump data dict  outfile          outfile write      else         Create file     with open fname   w   as outfile          array              array append data dict          json dump array  outfile

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import jsonlines  object1                     quot name quot    quot name1 quot                   quot url quot    quot url1 quot               object2                     quot name quot    quot name2 quot                   quot url quot    quot url2 quot                     filename jsonl is the name of the file with jsonlines open  quot filename jsonl quot    quot a quot   as writer      for writing     writer write object1      writer write object2   with jsonlines open  filename jsonl   as reader         for reading     for obj in reader          print obj                visit for more info https   jsonlines readthedocs io en latest

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One possible solution is do the concatenation manually  here is some useful  code   import json def append to json  dict path        with open path   ab    as f          f seek 0 2                                  Go to the end of file             if f tell      0                            Check if file is empty             f write json dumps   dict   encode      If empty  write an array         else               f seek -1 2                         f truncate                              Remove the last character  open the array             f write       encode                    Write the separator             f write json dumps  dict  encode        Dump the dictionary             f write     encode                      Close the array   You should be careful when editing the file outside the script not add any spacing at the end

User · Answer

Append entry to the file contents if file exists  otherwise append the entry to an empty list and write in in the file   a      if not os path isfile fname       a append entry      with open fname  mode  w   as f          f write json dumps a  indent 2   else      with open fname  as feedsjson          feeds   json load feedsjson       feeds append entry      with open fname  mode  w   as f          f write json dumps feeds  indent 2

User · Answer

You aren t ever writing anything to do with the data you read in  Do you want to be adding the data structure in feeds to the new one you re creating   Or perhaps you want to open the file in append mode open filename   a   and then add your string  by writing the string produced by json dumps instead of using json dump - but nneonneo points out that this would be invalid json

User · Answer

Using a instead of w should let you update the file instead of creating a new one overwriting everything in the existing file   See this answer for a difference in the modes

[python] How to append data to a json file?

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