Invalid type for parameter input (step_function.start_execution)

I have a start_execution command to start a step function via my lambda function (python):

    if event['Records'][0]['eventName'] == 'INSERT':
            filename, source, destination_bucket_name, filekey = parse_file_info_from_trigger(event)
            response = client.start_execution(
                stateMachineArn='aws:states:.......',
                input = "{\"first_name\" : \"test\"}"       
            )
    else: 
        logger.info(f'This is not an Insert event')

How can I pass the above extracted variables (filename, source etc) into the input of the start-execution command?

I tried this:

 response = step_function.start_execution(
                    stateMachineArn=state_machine_zip_files_arn,
                    input = str({ "filename": f"{filename}",  "filetype": f"{filetype}",  "unixtimestamp": f"{unixtimestamp}", 
                    "masterclient": f"{masterclient}", "source_bucket_name": f"{source_bucket_name}" , 
                    "destination_bucket_name": f"{destination_bucket_name}", "filekey": f"{filekey}","this is a test string": f"teststring"})     
                    )

but it gives me an error that:

Unable to start_execution for state machine: An error occurred (InvalidExecutionInput) when calling the StartExecution operation: Invalid State Machine Execution Input: 'Unexpected character (''' (code 39)): was expecting double-quote to start field name'

>Solution :

The expected format of input is a str. You should convert your dict into a str as following: json.dumps(your_input_data).