Count appearances of a value until it changes to another value

Use:

df = df.groupby(df['values'].ne(df['values'].shift()).cumsum())['values'].value_counts()

Or:

df = df.groupby([df['values'].ne(df['values'].shift()).cumsum(), 'values']).size()

---

print (df)
values  values
1       10        2
2       23        2
3       9         3
4       10        4
5       12        1
Name: values, dtype: int64

Last for remove first level:

df = df.reset_index(level=0, drop=True)
print (df)
values
10    2
23    2
9     3
10    4
12    1
dtype: int64

Explanation:

Compare original column by shifted with not equal ne and then add cumsum for helper Series:

print (pd.concat([df['values'], a, b, c], 
                 keys=('orig','shifted', 'not_equal', 'cumsum'), axis=1))
    orig  shifted  not_equal  cumsum
0     10      NaN       True       1
1     10     10.0      False       1
2     23     10.0       True       2
3     23     23.0      False       2
4      9     23.0       True       3
5      9      9.0      False       3
6      9      9.0      False       3
7     10      9.0       True       4
8     10     10.0      False       4
9     10     10.0      False       4
10    10     10.0      False       4
11    12     10.0       True       5

You can keep track of where the changes in df['values'] occur, and groupby the changes and also df['values'] (to keep them as index) computing the size of each group

changes = df['values'].diff().ne(0).cumsum()
df.groupby([changes,'values']).size().reset_index(level=0, drop=True)

 values
10    2
23    2
9     3
10    4
12    1
dtype: int64

itertools.groupby

from itertools import groupby

pd.Series(*zip(*[[len([*v]), k] for k, v in groupby(df['values'])]))

10    2
23    2
9     3
10    4
12    1
dtype: int64

---

It's a generator

def f(x):
  count = 1
  for this, that in zip(x, x[1:]):
    if this == that:
      count += 1
    else:
      yield count, this
      count = 1
  yield count, [*x][-1]

pd.Series(*zip(*f(df['values'])))

10    2
23    2
9     3
10    4
12    1
dtype: int64

Using crosstab

df['key']=df['values'].diff().ne(0).cumsum()
pd.crosstab(df['key'],df['values'])
Out[353]: 
values  9   10  12  23
key                   
1        0   2   0   0
2        0   0   0   2
3        3   0   0   0
4        0   4   0   0
5        0   0   1   0

Slightly modify the result above

pd.crosstab(df['key'],df['values']).stack().loc[lambda x:x.ne(0)]
Out[355]: 
key  values
1    10        2
2    23        2
3    9         3
4    10        4
5    12        1
dtype: int64

---

Base on python groupby

from itertools import groupby

[ (k,len(list(g))) for k,g in groupby(df['values'].tolist())]
Out[366]: [(10, 2), (23, 2), (9, 3), (10, 4), (12, 1)]