SQLite3 and multiple processes

- by mhdr

First, avoid concurrent access to sqlite database files. Concurrency is one of sqlite’s weak points and if you have a highly concurrent application, consider using another database engine.

If you cannot avoid concurrency or drop sqlite, wrap your write transactions in BEGIN IMMEDIATE; … END;. The default transaction mode in sqlite is DEFERRED which means that a lock is acquired only on first actual write attempt. With IMMEDIATE transactions, the lock is acquired immediately, or you get SQLITE_BUSY immediately. When someone holds a lock to the database, other locking attempts will result in SQLITE_BUSY.

Dealing with SQLITE_BUSY is something you have to decide for yourself. For many applications, waiting for a second or two and then retrying works quite all right, giving up after n failed attempts. There are sqlite3 API helpers that make this easy, e.g. sqlite3_busy_handler() and sqlite3_busy_timeout() but it can be done manually as well.

You could also use OS level synchronization to acquire a mutex lock to the database, or use OS level inter-thread/inter-process messaging to signal when one thread is done accessing the database.

If you want to use Python’s automatic transaction handling, leave isolation_level at its default value, or set it to one of the three levels.

If you want to do your own transaction handling, you have to prevent Python from doing its own by setting isolation_level to None.

References
https://stackoverflow.com/questions/1063438/sqlite3-and-multiple-processes
https://stackoverflow.com/questions/41915603/python3-sqlite3-begin-immediate-error

Extract .zip files in Inno Setup

- by mhdr

You can use an external command line tool for unzipping your archive, see here for example. Put it in your [Files] section:

[Files]
Source: "7za.exe"; DestDir: "{tmp}"; Flags: deleteafterinstall;

Then call it in your [Run] section, like this:

[Run]
Filename: {tmp}\7za.exe; Parameters: "x ""{tmp}\example.zip"" -o""{app}\"" * -r -aoa"; Flags: runhidden runascurrentuser;

References
https://stackoverflow.com/questions/40704442/is-there-a-way-to-extract-zip-files-in-inno-setup-after-a-certain-page-is-done
https://stackoverflow.com/questions/6065364/how-to-get-inno-setup-to-unzip-a-file-it-installed-all-as-part-of-the-one-insta

Processing Incoming Request Data in Flask

- by mhdr 
from flask import request

Query Arguments

@app.route('/query-example')
def query_example():
    language = request.args.get('language') #if key doesn't exist, returns None

    return '''<h1>The language value is: {}</h1>'''.format(language)

Form Data

@app.route('/form-example', methods=['GET', 'POST']) #allow both GET and POST requests
def form_example():
    if request.method == 'POST':  #this block is only entered when the form is submitted
        language = request.form.get('language')
        framework = request.form['framework']

        return '''<h1>The language value is: {}</h1>
                  <h1>The framework value is: {}</h1>'''.format(language, framework)

    return '''<form method="POST">
                  Language: <input type="text" name="language"><br>
                  Framework: <input type="text" name="framework"><br>
                  <input type="submit" value="Submit"><br>
              </form>'''

JSON Data

@app.route('/json-example', methods=['POST']) #GET requests will be blocked
def json_example():
    req_data = request.get_json()

    language = req_data['language']
    framework = req_data['framework']
    python_version = req_data['version_info']['python'] #two keys are needed because of the nested object
    example = req_data['examples'][0] #an index is needed because of the array
    boolean_test = req_data['boolean_test']

    return '''
           The language value is: {}
           The framework value is: {}
           The Python version is: {}
           The item at index 0 in the example list is: {}
           The boolean value is: {}'''.format(language, framework, python_version, example, boolean_test)

References
https://www.digitalocean.com/community/tutorials/processing-incoming-request-data-in-flask