A function in pandas or numpy to filter column by list of values that follows it

df = pd.DataFrame({"changes":['increase', 'constant', 'constant', 'constant', 'decline', 'constant', 'constant', 'increase', 'constant', 'constant', 'constant','decline', 'constant', 'constant', 'constant']})

### Build group
df['group'] = df['changes'].ne(df['changes'].shift()).cumsum()
df
###
     changes  group
0   increase      1
1   constant      2
2   constant      2
3   constant      2
4    decline      3
5   constant      4
6   constant      4
7   increase      5
8   constant      6
9   constant      6
10  constant      6
11   decline      7
12  constant      8
13  constant      8
14  constant      8

Create masks to filter out unwanted data

mask_1 = df['changes'].eq('decline') & df['changes'].shift(-1).eq('constant')
mask_2 = df['changes'].eq('constant') & df['changes'].shift().eq('decline')
groups = df.loc[mask_1 | mask_2, 'group']
groups

It indicates group 3,4,7,8 should be excluded

Assign filtered data to result

result = df[~df['group'].isin(groups)].drop(columns=['group'])
result
###
    changes
0  increase
1  constant
2  constant
3  constant
4  increase
5  constant
6  constant
7  constant