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# First usage of Python libraries

Most Python code uses third party libraries. During this course, we will learn how to
install such libraries and how to use few popular third party libraries. However, during
all the first chapters to learn Python, we won't use such libraries.

In this notebook, we show few examples of nice and simple things that can be done with
few popular libraries.

## Matplotlib example

Let us start with a simple Matplotlib/Pandas plot (taken from the Matplotlib gallery,
https://matplotlib.org/3.1.1/gallery/lines_bars_and_markers/errorbar_subsample.html).

```{code-cell}
import numpy as np
import matplotlib.pyplot as plt

# example data
x = np.arange(0.1, 4, 0.1)
y = np.exp(-x)

# example variable error bar values
yerr = 0.1 + 0.1 * np.sqrt(x)

# Now switch to a more OO interface to exercise more features.
fig, axs = plt.subplots(nrows=1, ncols=2, sharex=True)
ax = axs[0]
ax.errorbar(x, y, yerr=yerr)
ax.set_title("all errorbars")

ax = axs[1]
ax.errorbar(x, y, yerr=yerr, errorevery=5)
ax.set_title("only every 5th errorbar")

fig.suptitle("Errorbar subsampling for better appearance");
```

## Vega-Altair example

We now switch to another example using another plotting library
[Vega-Altair](https://altair-viz.github.io/), more oriented on producing interactive web
plots.

```{code-cell}
:tags: [skip-execution]

import altair as alt

# load a sample dataset as a pandas DataFrame
from vega_datasets import data
```

```{code-cell}
:tags: [skip-execution]

cars = data.cars()

# make the chart
alt.Chart(cars).mark_point().encode(
    x="Horsepower",
    y="Miles_per_Gallon",
    color="Origin",
).interactive()
```

## Ipyleaflet example

Finally, we show a last example using [ipyleaflet](https://ipyleaflet.readthedocs.io):

```{code-cell}
:tags: [skip-execution]

from ipyleaflet import Map, Marker

center = (52.204793, 360.121558)
map = Map(center=center, zoom=12)

# Add a draggable marker to the map
# Dragging the marker updates the marker.location value in Python
marker = Marker(location=center, draggable=True)
map.add_control(marker)

display(map)
```

```{code-cell}
:tags: [skip-execution]

# We can also update the marker location from Python
marker.location = (52.2, 360.1)
```

```{code-cell}
:tags: [skip-execution]

# We can run a python function when the marker location changes
# Here we'll adjust the center of the map to follow the marker
def on_location_changed(value):
    map.center = value.new


# Call the on_location_changed function when marker.location changes
marker.observe(on_location_changed, "location")
```