import pandas as pd pd.set_option('display.mpl_style', 'default') figsize(15, 3)
We saw earlier that pandas is really good at dealing with dates. It is also amazing with strings! We're going to go back to our weather data from Chapter 5, here.
weather_2012 = pd.read_csv('../data/weather_2012.csv', parse_dates=True, index_col='Date/Time') weather_2012[:5]
|Temp (C)||Dew Point Temp (C)||Rel Hum (%)||Wind Spd (km/h)||Visibility (km)||Stn Press (kPa)||Weather|
|2012-01-01 02:00:00||-1.8||-3.4||89||7||4.0||101.26||Freezing Drizzle,Fog|
|2012-01-01 03:00:00||-1.5||-3.2||88||6||4.0||101.27||Freezing Drizzle,Fog|
You'll see that the 'Weather' column has a text description of the weather that was going on each hour. We'll assume it's snowing if the text description contains "Snow".
pandas provides vectorized string functions, to make it easy to operate on columns containing text. There are some great examples in the documentation.
weather_description = weather_2012['Weather'] is_snowing = weather_description.str.contains('Snow')
This gives us a binary vector, which is a bit hard to look at, so we'll plot it.
# Not super useful is_snowing[:5]
Date/Time 2012-01-01 00:00:00 False 2012-01-01 01:00:00 False 2012-01-01 02:00:00 False 2012-01-01 03:00:00 False 2012-01-01 04:00:00 False Name: Weather, dtype: bool
# More useful! is_snowing.plot()
<matplotlib.axes.AxesSubplot at 0x403c190>
If we wanted the median temperature each month, we could use the
resample() method like this:
weather_2012['Temp (C)'].resample('M', how=np.median).plot(kind='bar')
<matplotlib.axes.AxesSubplot at 0x560cc50>
Unsurprisingly, July and August are the warmest.
So we can think of snowiness as being a bunch of 1s and 0s instead of
Date/Time 2012-01-01 00:00:00 0 2012-01-01 01:00:00 0 2012-01-01 02:00:00 0 2012-01-01 03:00:00 0 2012-01-01 04:00:00 0 2012-01-01 05:00:00 0 2012-01-01 06:00:00 0 2012-01-01 07:00:00 0 2012-01-01 08:00:00 0 2012-01-01 09:00:00 0 Name: Weather, dtype: float64
and then use
resample to find the percentage of time it was snowing each month
Date/Time 2012-01-31 0.240591 2012-02-29 0.162356 2012-03-31 0.087366 2012-04-30 0.015278 2012-05-31 0.000000 2012-06-30 0.000000 2012-07-31 0.000000 2012-08-31 0.000000 2012-09-30 0.000000 2012-10-31 0.000000 2012-11-30 0.038889 2012-12-31 0.251344 Freq: M, dtype: float64
<matplotlib.axes.AxesSubplot at 0x5bdedd0>
So now we know! In 2012, December was the snowiest month. Also, this graph suggests something that I feel -- it starts snowing pretty abruptly in November, and then tapers off slowly and takes a long time to stop, with the last snow usually being in April or May.
We can also combine these two statistics (temperature, and snowiness) into one dataframe and plot them together:
temperature = weather_2012['Temp (C)'].resample('M', how=np.median) is_snowing = weather_2012['Weather'].str.contains('Snow') snowiness = is_snowing.astype(float).resample('M', how=np.mean) # Name the columns temperature.name = "Temperature" snowiness.name = "Snowiness"
concat again to combine the two statistics into a single dataframe.
stats = pd.concat([temperature, snowiness], axis=1) stats
<matplotlib.axes.AxesSubplot at 0x5f59d50>
Uh, that didn't work so well because the scale was wrong. We can do better by plotting them on two separate graphs:
stats.plot(kind='bar', subplots=True, figsize=(15, 10))
array([<matplotlib.axes.AxesSubplot object at 0x5fbc150>, <matplotlib.axes.AxesSubplot object at 0x60ea0d0>], dtype=object)