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Helper to create fake filesystem and quick capture its state (or state of a real one).

Project description

FsForge - file system tests helper

fsforge - is a tool set defining fake or real file-system layout.

Gives among others a functionality:

  • for creating artificial fs using pyfakefs.
  • for taking a snapshot of either real or faked file system.

In short, it is some kind of syntax' extension to pyfakefs and is intended to use with pytest framework. Allows for absolute transparency in fs operations, so that any kind of tests: unit, functional or end-to-end can be performed in memory - instead of real hard disc operations (SSDs can breathe and relax), without any headache nor enormous setup nor teardown.

The main difference against bare pyfakefs is that fsforge uses nice and clean dict literals instead of lists of paths. It also allows for reverse operation - to create the same kind of nested dictionary structure defining given fs with just a single function call.

Such a result is immediately ready to make assertions on it.

Works with python 3.6, 3.7, 3.8, 3.9, 3.10 and pypy3. Requires pyfakefs==4.5.*.


Capture real or faked fs snapshot.

Let's use the following structure originated in /tmp/ex for all further examples:

 bash>$ tree /tmp/ex
 ├── dir_a
 │   ├── sub_empty_dir
 │   ├── sub_dir_with_a_file
 │   │   ├── app_dump.json
 │   │   └── file_1.txt
 │   └── file_2.txt
 ├── dir_b
 │   ├── special_file.txt
 │   └── file_4.bin
 ├── empty_dir
 └── special_file_2.txt

We can collect a snapshot of this layout with:

import pprint
from fsforge import take_fs_snapshot

tree = take_fs_snapshot('/tmp/ex')

will output such a tree:

tree = {
     'dir_a': {
         'sub_empty_dir': {},
         'sub_dir_with_a_file': {
             'app_dump.json': None,
             'file_1.txt': None
         'file_2.txt': None,
     'dir_b': {
         'app_dump.json': None,
         'special_file.txt': None,
         'file_4.bin': None
     'empty_dir': {},
     'special_file_2.txt': None,


The resulting tree is a pure dict. fsforge uses magic relation and similarity of directory to a python's dictionary.

Directories are recognized by being dict instances. Any other value type in the tree is treat as a file. fsforge distinguishes only dict (as directories) and non dict (files) while traversing the tree.

Create forged file system

Now fsforge can use such kind of tree to perform needed pyfakefs' calls to recreate the structure in memory for some pytest tests:

import os

from fsforge import create_fs

def test_that(fs):

    create_fs(fs, tree, "/tmp/ex")

    # everything is now set up:
    assert os.path.isdir("/tmp/ex/dir_a/sub_dir_with_a_file")
    assert os.path.isfile("/tmp/ex/dir_a/file_2.txt")

In the code above:

  • fs is a fixture automatically accessible in tests as soon as you have pyfakefs package installed. It can also be a FakeFilesystem object instance imported from pyfakefs.
  • fs could also be a fsforge.RealFS object. Real writes will be performed.
  • tree is reused dictionary from previous code snippets
  • "/tmp/ex" is a origin of "mount point" of given structure, pyfakefs will anchor items specified in tree to this path.

Basic Workflow

The application you test may make some changes to given file system. After some time you can collect a snapshot of the fs structure and make needed assertions on the changes made to its state.

E.g. Probably some files were removed or created, some content appended. Whatever... Of course, you may not be interested with all of that, that's why there is:

File Processors and Path Masking

The take_fs_snapshot function takes a file system mask definition as an argument. File processor is just a function provided by you taking a given file path and returning anything you need from that file, e.g. it's contents (or any processing result or None).

import json

def reader(file_path):
    with open(file_path, 'r') as file_:

def json_reader(file_path):
    content = json.loads(reader(file_path))
    return json.dumps(content["some section only"])

Assume we have a file system from /tmp/ex from the beginning of this README. And we want to read:

  • whole contents of any file in dir_b whose name contains file substring
  • app_dump.json - in whatever directory but "some section only" is interesting
  • note existence of any files in the top dir
  • ignore existence of any other file

So let's create a mask and call it:

from fsforge import iddle_file_processor

# iddle_file_processor returns None regardless of call argument, is used to note
# files existence (without that file is ignored and does not appear in the result tree)

mask = {
    'dir_b': {
        # any file containing 'file' substring
        '*file*': reader,
    '**': {
        # any file named app_dump.json in whatever path
        'app_dump.json': json_reader,
    # Note any file in top level directory (but don't read it)
    '*': iddle_file_processor,

result = take_fs_snapshot("/tmp/ex", mask)

from pprint import pprint


    'dir_a': {
        'sub_empty_dir': {},
        'sub_dir_with_a_file': {
            'app_dump.json': '{"some section only": "its contents"}',
    'dir_b': {
        'app_dump.json': '{"some section only": "its contents in dir_b"},
        'special_file.txt': "distinguished content",
        'file_4.bin': "contents of file_4"
    'empty_dir': {},
    'special_file_2.txt': None,

Does that result look similar? Yes, it's the same kind of tree, but files have strings instead of None. It can be used to recreate the fs with these strings as contents of the new files.

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