Using blob.upload_from_filename("dataframe.csv") gets OSError: [Errno 30] Read-only file system: 'model_label.csv'

Question

I am trying to use Google Cloud Function to query from bigquery and save a dataframe into a file. I've tried the code with the Jupyter that comes with GCP and it's a success. However, when I copy the code to Cloud Function and test it. I got an error message as following:

OSError: [Errno 30] Read-only file system: 'dataframe.csv'

My code can be found below:

def save_dataframe(dataframe, filename):
    client = storage.Client(project="my-first-project")
    bucket = client.get_bucket("test_bucket")
    dataframe.to_csv(filename)
    blob = bucket.blob(filename)
    blob.upload_from_filename(filename)

save_dataframe(dataframe, "dataframe.csv")

I have another function which works fine:

def save_model(models, filename):
    client = storage.Client(project="my-first-project")
    bucket = client.get_bucket("test_bucket")
    blob = bucket.blob(filename)
    pickle_out = pickle.dumps(models)
    blob.upload_from_string(pickle_out)