"""Download latest time series data from BOM website and update local copy.

BOM does not allow direct http requests to data files unless it believes the
request comes from a browser. For example:

>>> pd.read_csv(url)  # This does not work
HTTPError: HTTP Error 403: Forbidden

>>> requests.get(url)  # This works!
<Response [200]>

D. Howe
2019-09-04
"""
import os
import json
import requests
import pandas as pd

# Set json product URL for North Head
# http://www.bom.gov.au/products/IDN60901/IDN60901.95768.shtml
json_url = 'http://www.bom.gov.au/fwo/IDN60901/IDN60901.95768.json'

# Set output directory
output_dir = 'bom'

# Set output column names
cols = [
    'wind_dir',
    'wind_spd_kmh',
    'gust_kmh',
]


def update_master(output_dir, csv_name, df):
    """Update master csv time series.

    Args:
        output_dir (str): path to time series directory
        csv_name (str):   name of time series file
        df (dataframe):   dataframe with datetime index

    Returns:
        None
    """
    try:
        # Load local master table if it exists
        master = pd.read_csv(os.path.join(output_dir, csv_name),
                             index_col=0,
                             parse_dates=True)

        # Only include timestamps that do not already exist
        df = df[~df.index.isin(master.index)]

        # Update master
        master = master.append(df)

    except FileNotFoundError:
        # Create new master table if none exists
        master = df

    # Export master table
    master.to_csv(os.path.join(output_dir, csv_name))


# Download JSON data
json_data = json.loads(requests.get(json_url).content)

# Create dataframe
df = pd.DataFrame(json_data['observations']['data'])

# Set local time as index
df.index = pd.to_datetime(df['local_date_time_full'])
df = df.sort_index()

# Extract columns of interest
df = df[cols]

# Update master table
csv_name = json_url.split('/')[-1].replace('.json', '.csv')
update_master(output_dir, csv_name, df)