Plot time windows based on interested value

Question

I use the following code to identify some interested values into a dataframe and them plot a time window before and after that value appeared. It works very well, but I would like to know if there is a less coding way/more pythonic way to accomplish this. Thanks in advance!

Before I go, I try to use only Seaborn on the plotting secction, but creating the subplots and filling then was easier, considering I don't want to share axis.

# Libraries
import numpy as np
import pandas as pd
import seaborn as sns
import matplotlib as mpl
import matplotlib.pyplot as plt
# Generate Data
rng = np.random.default_rng(12345)

df = pd.DataFrame(
    index=pd.date_range(start="1/1/2019", periods=1035, freq='D'),
    data={'value':rng.integers(-100, 30000, 1035)}
).reset_index()

# Creating a boolean for interesting values
df['select'] = df['value'] < 0

# Finding days with interested value and creating the periods
lt_dates = df.loc[df['select'], 'index'].to_list()
lt_days_after = [pd.DataFrame(index=pd.date_range(start=day, periods=14, freq='D')).reset_index() for day in lt_dates]
lt_days_before = [pd.DataFrame(index=pd.date_range(end=day, periods=14, freq='D')).reset_index() for day in lt_dates]

# Concatenating the periods
df_mask = pd.concat(objs=[pd.concat(lt_days_after), pd.concat(lt_days_before)]).sort_values('index').drop_duplicates(ignore_index=True)

# Flagging days
df['grouped'] = df['index'].isin(df_mask['index'])

# Creating the groups
df['slice'] = (~df['grouped']).cumsum()
groups = df.loc[df['grouped'], 'slice'].unique()
groups_dict = {y: x for x, y in enumerate(groups)}

# Filtering non interested values
df = df.loc[df['grouped']]

df['slice'].replace(groups_dict, inplace=True)

# Plotting
rows = len(groups_dict)

# Function to fill the subplots
def subplot_df(_ax, x):
        sns.lineplot(
            x="index",
            y='value',
            ci=None,
            data=df[df['slice'] == x],
            ax=_ax
        )
        _ax.set_xlabel('')

f, ax = plt.subplots(
    nrows=rows,
    figsize=(12, 2*rows),
    sharex=False,
    sharey=False
)

# Filling a single or multiple subplots
if rows != 0:
    for x in range(rows):
        subplot_df(ax[x], x)
else:
    subplot_df(ax, 0)

Answer 1

Avoid passing strings for parameters like start when they come from application constants and not the user; use date instances instead.

You rely on reset_index too much - this can go away and be replaced with direct construction of the dataframe. One consequence of your approach is that there's a column called index, which is not a good idea because attribute access for the column is broken (i.e. you can't write df.index).

Your use of isin, the separate treatment of after and before, and your enumerate and replace can go away. Consider instead a broadcast comparison of time deltas.

Suggested

from datetime import date

import matplotlib.pyplot as plt
import numpy as np
import pandas as pd
import seaborn as sns


rng = np.random.default_rng(12345)

df = pd.DataFrame({
    'when': pd.date_range(
        start=date(2019, 1, 1), periods=1035, freq='D'
    ),
    'value': rng.integers(low=-100, high=30_000, size=1035)
})

# Creating a boolean for interesting values
selected = df.value < 0

all_selected = np.abs(
    df['when'].values[:, np.newaxis] -
    df['when'][selected].values[np.newaxis, :]
) < pd.to_timedelta(14, unit='D')

df['grouped'] = np.bitwise_or.reduce(all_selected, axis=1)
grouped = np.zeros(1+len(selected), dtype=bool)
grouped[1:] = df.grouped
changes = np.abs(np.diff(grouped))
df['slice'] = changes.cumsum() // 2

# Filtering non interesting values
df = df.loc[df.grouped, :]

# Plotting
rows = round(changes.sum() // 2)

# Function to fill the subplots
def subplot_df(ax: plt.Axes, x: int) -> None:
    sns.lineplot(
        x='when',
        y='value',
        ci=None,
        data=df[df['slice'] == x],
        ax=ax,
    )
    ax.set_xlabel('')


f, ax = plt.subplots(
    nrows=rows,
    figsize=(12, 2*rows),
    sharex=False,
    sharey=False,
)

# Filling a single or multiple subplots
if rows == 0:
    subplot_df(ax, 0)
else:
    for x in range(rows):
        subplot_df(ax[x], x)

plt.show()