How to decode/read Flask filesystem session files?

Question

I have a Python3 Flask app using Flask-Session (which adds server-side session support) and configured to use the filesystem type.

This type underlying uses the Werkzeug class werkzeug.contrib.cache.FileSystemCache (Werkzeug cache documentation).

The raw cache files look like this if opened:

J¬».].Äï;}î(å
_permanentîàå
respondentîåuuidîåUUIDîìî)Åî}î(åintîät˙ò∑flŒºçLÃ/∆6jhåis_safeîhåSafeUUIDîìîNÖîRîubåSECTIONS_VISITEDî]îåcurrent_sectionîKåSURVEY_CONTENTî}î(å0î}î(ås_idîås0îånameîåWelcomeîådescriptionîåîå   questionsî]î}î(ås_idîhåq_idîhåq_constructîhåq_textîhå
q_descriptionîhåq_typeîhårequiredîhåoptions_rowîhåoptions_row_alpha_sortîhåreplace_rowîhåoptions_colîhåoptions_col_codesîhåoptions_col_alpha_sortîhåcond_continue_rules_rowîhåq_meta_notesîhuauå1î}î(hås1îhå    Screeningîhå[This section determines if you fit into the target group.îh]î(}î(hh/håq1îh hh!å9Have you worked on a product in this field before?

The items stored in the session can be seen a bit above: - current_section should be an integer, e.g., 0 - SECTIONS_VISITED should be an array of integers, e.g., [0,1,2] - SURVEY_CONTENT format should be an object with structure like below

{
  'item1': {
    'label': string,
    'questions': [{}]
  }, 
  'item2': {
    'label': string,
    'questions': [{}]
  }
}

What you can see in the excerpt above, for example the text This section determines if you fit into the target group is the value of one label. The stuff after questions are keys that can be found in each questions object, e.g., q_text as well as their values, e.g., Have you worked on a product in this field before? is the value of q_text.

I need to retrieve data from the stored cache files in a way that I can read them without all the extra characters like å.

I tried using Werkzeug like this, where the item 9c3c48a94198f61aa02a744b16666317 is the name of the cache file I want to read. However, it was not found in the cache directory.

from werkzeug.contrib.cache import FileSystemCache
cache_dir="flask_session"
mode=0600
threshold=20000
cache = FileSystemCache(cache_dir, threshold=threshold, mode=mode)
item = "9c3c48a94198f61aa02a744b16666317"
print(cache.has(item))
data = cache.get(item)
print(data)

What ways are there to read the cache files?

I opened a GitHub issue in Flask-Session, but that's not really been actively maintained in years.

For context, I had an instance where for my web app writing to the database was briefly not working - but the data I need was also being saved in the session. So right now the only way to retrieve that data is to get it from these files.

EDIT:

Thanks to Tim's answer I solved it using the following:

import pickle
obj = []
with open(file_name,"rb") as fileOpener:
    while True:
        try:
            obj.append(pickle.load(fileOpener))
        except EOFError:
            break
print(obj)

I needed to load all pickled objects in the file, so I combined Tim's solution with the one here for loading multiple objects: https://stackoverflow.com/a/49261333/11805662

Without this, I was just seeing the first pickled item.

Also, in case anyone has the same problem, I needed to use the same python version as my Flask app (related post). If I didn't, then I would get the following error:

ValueError: unsupported pickle protocol: 4

Answer 1

You can decode the data with pickle. Pickle is part of the Python standard library.

import pickle

with open("PATH/TO/SESSION/FILE") as f:
    data = pickle.load(f)