Is pandas.core.groupby.GroupBy.agg an efficient solution for calculating multiple columns by group?

Question

Suppose you have a pandas.DataFrame like this one:

import pandas as pd
import numpy as np
from math import exp, log10

df = pd.DataFrame({'S' : ['C', 'A', 'A', 'B', 'B', 'A'],
                   'ID': ['ID3', 'ID1', 'ID2', 'ID4', 'ID4', 'ID1'],
                   'M' : ['Y', 'X', 'X', 'Y', 'Z', 'X'],                   
                   'V' : ['<1', '<0.5', 3, '>10', 7, 6]
                })

df

#   S   ID  M     V
#0  C  ID3  Y    <1
#1  A  ID1  X  <0.5
#2  A  ID2  X     3
#3  B  ID4  Y   >10
#4  B  ID4  Z     7
#5  A  ID1  X     6

and a dictionary that maps each M to a function of x, like:

M_to_transf_dict = {'X' : 'x', 'Y' : 'exp(x)', 'Z' : 'log10(x)'}

This is what you need to do:

• split V into the numerical part ( V_u ) and qualifier ( V_c : '<', '>', if not already numerical or interpretable as such)
• group by S, M , and for each group:
  • calculate the mean V_mean , count V_N and sample standard deviation V_sample_sd of V_u
  • make the comma-separated list of unique ID 's and store the result into ID
  • find the most frequent '<' or '>' qualifier in V_c and store the result into V_mean_c
  • apply the function of x corresponding to each M to V_mean and store the result into TV_mean
  • apply V_mean_c on V_mean and TV_mean , when '<' or '>'

After many trials, I put together something that seems to work, but I have doubts regarding performance.
I saw some posts (eg this one ) questioning the use of .apply , and indeed from some tests it appears that handling the splitting of V as an external map followed by column assignment is much faster than .apply :

def unc(v):
    try:
        u = float(v)
        c = "="
    except ValueError:
        v = str(v)
        u = float(v[1:])
        c = v[0]
    except:
        u = pd.NA
        c = "="
    return [u, c]

%timeit df[['V_u', 'V_c']] = df.apply(lambda row : unc(row['V']), axis = 1, result_type = 'expand')
# 698 µs ± 5.22 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

%%timeit
uc = df['V'].map(unc)
u = [uci[0] for uci in uc.values]
c = [uci[1] for uci in uc.values]
df['V_u'] = u
df['V_c'] = c
# 129 µs ± 3.59 µs per loop (mean ± std. dev. of 7 runs, 10,000 loops each)

%%timeit
uc = df['V'].map(unc)
u, c = [], []
for uci in uc:
    u.append(uci[0])
    c.append(uci[1])
df['V_u'] = u
df['V_c'] = c
# 124 µs ± 1.11 µs per loop (mean ± std. dev. of 7 runs, 10,000 loops each)

(Note that I remade df before each timeit ).

For the other operations I described, except the last two, I used .groupby.agg with named aggregation:

def majority(c_list):
    c_list = list(c_list)
    Nlt = c_list.count('<')
    Ngt = c_list.count('>')
    if Nlt + Ngt == 0:
        c = '='
    else:
        c = sorted(zip([Nlt, Ngt], ['<','>']))[1][1]
    return c

%%timeit
df_summary_by_S_M = df.groupby(['S','M'], as_index = False).agg(
                    ID = pd.NamedAgg(column = 'ID', aggfunc = lambda x : ','.join(np.unique(x))),
                    V_mean = pd.NamedAgg(column = 'V_u', aggfunc = 'mean'),
                    V_mean_c = pd.NamedAgg(column = 'V_c', aggfunc = majority),
                    V_N = pd.NamedAgg(column = 'V_u', aggfunc = 'count'),
                    V_sample_sd = pd.NamedAgg(column = 'V_u', aggfunc = 'std')
)
# 4.17 ms ± 61.1 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

df_summary_by_S_M

#   S  M       ID     V_mean V_mean_c  V_N  V_sample_sd
#0  A  X  ID1,ID2   3.166667        <    3     2.753785
#1  B  Y      ID4  10.000000        >    1          NaN
#2  B  Z      ID4   7.000000        =    1          NaN
#3  C  Y      ID3   1.000000        <    1          NaN

I don't know how to place this timing, especially because I imagine .agg is a form of .apply , so I might not be using the best possible approach, having seen the previous results.
On a real dataset I am handling, with about 450 K records that are reduced to 230 K by grouping, this aggregation takes about 1 minute.
I am going to have to handle much larger datasets, and this might become an issue.

Would anyone be able to suggest a more efficient/performant approach to do the grouped calculations I described?
Or is this thought to be state-of-the-art performance for this kind of operation? I really have no benchmarks to judge that, hence my asking the question here.

For the last two steps, I think I will use list comprehensions, like:

%timeit df_summary_by_S_M['TV_mean'] = [eval(t) for x, t in \
    zip(df_summary_by_S_M['V_mean'], df_summary_by_S_M['M'].map(M_to_transf_dict))]
# 326 µs ± 5.06 µs per loop (mean ± std. dev. of 7 runs, 1,000 loops each)

df_summary_by_S_M

#   S  M       ID     V_mean V_mean_c  V_N  V_sample_sd       TV_mean
#0  A  X  ID1,ID2   3.166667        <    3     2.753785      3.166667
#1  B  Y      ID4  10.000000        >    1          NaN  22026.465795
#2  B  Z      ID4   7.000000        =    1          NaN      0.845098
#3  C  Y      ID3   1.000000        <    1          NaN      2.718282

and:

df_summary_by_S_M['V_mean'] = [c + str(v) if c != '=' else v for c, v in \
    zip(df_summary_by_S_M['V_mean_c'], df_summary_by_S_M['V_mean'])]

df_summary_by_S_M['TV_mean'] = [c + str(v) if c != '=' else v for c, v in \
    zip(df_summary_by_S_M['V_mean_c'], df_summary_by_S_M['TV_mean'])]

df_summary_by_S_M

#   S  M       ID               V_mean V_mean_c  V_N  V_sample_sd  \
#0  A  X  ID1,ID2  <3.1666666666666665        <    3     2.753785   
#1  B  Y      ID4                >10.0        >    1          NaN   
#2  B  Z      ID4                  7.0        =    1          NaN   
#3  C  Y      ID3                 <1.0        <    1          NaN   

#               TV_mean  
#0  <3.1666666666666665  
#1  >22026.465794806718  
#2             0.845098  
#3   <2.718281828459045  

Again, unless someone can suggest a more efficient alternative(?).

---

EDIT trying out @mozway's code

Original code, all operations timed together.

import pandas as pd
import numpy as np
from math import exp, log10

df0 = pd.DataFrame({'S' : ['C', 'A', 'A', 'B', 'B', 'A'],
                   'ID': ['ID3', 'ID1', 'ID2', 'ID4', 'ID4', 'ID1'],
                   'M' : ['Y', 'X', 'X', 'Y', 'Z', 'X'],                   
                   'V' : ['<1', '<0.5', 3, '>10', 7, 6]
                })

M_to_transf_dict = {'X' : 'x', 'Y' : 'exp(x)', 'Z' : 'log10(x)'}

def unc(v):
    try:
        u = float(v)
        c = "="
    except ValueError:
        v = str(v)
        u = float(v[1:])
        c = v[0]
    except:
        u = pd.NA
        c = "="
    return [u, c]

def majority(c_list):
    c_list = list(c_list)
    Nlt = c_list.count('<')
    Ngt = c_list.count('>')
    if Nlt + Ngt == 0:
        c = '='
    else:
        c = sorted(zip([Nlt, Ngt], ['<','>']))[1][1]
    return c

%%timeit
df = df0.copy()

uc = df['V'].map(unc)
u, c = [], []
for uci in uc:
    u.append(uci[0])
    c.append(uci[1])
df['V_u'] = u
df['V_c'] = c

df = df.groupby(['S','M'], as_index = False).agg(
                    ID = pd.NamedAgg(column = 'ID', aggfunc = lambda x : ','.join(np.unique(x))),
                    V_mean = pd.NamedAgg(column = 'V_u', aggfunc = 'mean'),
                    V_mean_c = pd.NamedAgg(column = 'V_c', aggfunc = majority),
                    V_N = pd.NamedAgg(column = 'V_u', aggfunc = 'count'),
                    V_sample_sd = pd.NamedAgg(column = 'V_u', aggfunc = 'std')
)

df['TV_mean'] = [eval(t) for x, t in \
    zip(df['V_mean'], df['M'].map(M_to_transf_dict))]
df['V_mean'] = [c + str(v) if c != '=' else v for c, v in \
    zip(df['V_mean_c'], df['V_mean'])]
df['TV_mean'] = [c + str(v) if c != '=' else v for c, v in \
    zip(df['V_mean_c'], df['TV_mean'])]

Result:

# 5.29 ms ± 100 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

@mozway's code.

def majority(s):
    mode = s.mode()
    return '=' if len(mode)>1 else mode.iloc[0]

M_dic = {'Y' : np.exp, 'Z' : np.log10}

df = df0.copy()

%%timeit
(df
 .join(df['V']
       .str.extract('(?P<V_c>\D)?(?P<V_u>\d+(?:\.\d+)?)')
       .astype({'V_u': float}).fillna({'V_c': '=', 'V_u': df['V']})
      )
 .assign(TV=lambda d: d.groupby('M')['V_u'].apply(lambda g: M_dic[g.name](g) if g.name in M_dic else g))
 .groupby(['S','M'], as_index = False)
 .agg(**{'ID': ('ID', lambda x: ','.join(x.unique())),
         'V_mean': ('V_u', 'mean'),
         'V_mean_c': ('V_c', majority), ## FIXME
         'V_N': ('V_u', 'count'),
         'V_sample_sd': ('V_u', 'std'),
         'TV_mean': ('TV', 'mean'),
 })
 .assign(TV_mean=lambda d: d['V_mean_c'].mask(d['V_mean_c'].eq('='), '')+d['TV_mean'].astype(str))
)

Result:

# 8.05 ms ± 259 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

---

EDIT 2 trying again, on a more realistic simulated dataset

import pandas as pd
import numpy as np
from numpy.random import default_rng
from math import exp, log10

# Simulate dataset to process
rng = default_rng(12345)

S = rng.choice(range(250000), 450000)
ID = S.copy()
S = [f"S_{Si}" for Si in S]
ID = [f"ID_{IDi}" for IDi in ID]
pM = [np.sqrt(1+i) for i in range(100)]
pM = pM / np.sum(pM)
M = rng.choice(range(100), 450000, p = pM)
M_to_transf_dict = dict()
for i in range(0,10):
    M_to_transf_dict[f"M_{i}"] = 'exp(x)'
for i in range(10,30):
    M_to_transf_dict[f"M_{i}"] = 'x'
for i in range(30,100):
    M_to_transf_dict[f"M_{i}"] = 'log10(x)'
M = [f"M_{Mi}" for Mi in M]
V = rng.random(450000)
Q = rng.choice(['', '<', '>'], 450000, p = [0.9, 0.05, 0.05])
V = [f"{q}{v}" for q, v in zip(Q, V)]
df0 = pd.DataFrame({'S' : S, 'ID' : ID, 'M' : M, 'V' : V})

Original code:

def unc(v):
    try:
        u = float(v)
        c = "="
    except ValueError:
        v = str(v)
        u = float(v[1:])
        c = v[0]
    except:
        u = pd.NA
        c = "="
    return [u, c]

def majority(c_list):
    c_list = list(c_list)
    Nlt = c_list.count('<')
    Ngt = c_list.count('>')
    if Nlt + Ngt == 0:
        c = '='
    else:
        c = sorted(zip([Nlt, Ngt], ['<','>']))[1][1]
    return c

%%timeit
df = df0.copy()

uc = df['V'].map(unc)
u, c = [], []
for uci in uc:
    u.append(uci[0])
    c.append(uci[1])
df['V_u'] = u
df['V_c'] = c

df = df.groupby(['S','M'], as_index = False).agg(
                    ID = pd.NamedAgg(column = 'ID', aggfunc = lambda x : ','.join(np.unique(x))),
                    V_mean = pd.NamedAgg(column = 'V_u', aggfunc = 'mean'),
                    V_mean_c = pd.NamedAgg(column = 'V_c', aggfunc = majority),
                    V_N = pd.NamedAgg(column = 'V_u', aggfunc = 'count'),
                    V_sample_sd = pd.NamedAgg(column = 'V_u', aggfunc = 'std')
)

df['TV_mean'] = [eval(t) for x, t in \
    zip(df['V_mean'], df['M'].map(M_to_transf_dict))]
df['V_mean'] = [c + str(v) if c != '=' else v for c, v in \
    zip(df['V_mean_c'], df['V_mean'])]
df['TV_mean'] = [c + str(v) if c != '=' else v for c, v in \
    zip(df['V_mean_c'], df['TV_mean'])]

Result:

# 20 s ± 289 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

@mozway's code ( M_dic changed according to the new set of M 's):

def majority(s):
    mode = s.mode()
    return '=' if len(mode)>1 else mode.iloc[0]

M_dic = dict()
for k in M_to_transf_dict:
    if M_to_transf_dict[k] == 'log10(x)':
        M_dic[k] = np.log10
    elif M_to_transf_dict[k] == 'exp(x)':
        M_dic[k] = np.exp

df = df0.copy()

%%timeit
(df
 .join(df['V']
       .str.extract('(?P<V_c>\D)?(?P<V_u>\d+(?:\.\d+)?)')
       .astype({'V_u': float}).fillna({'V_c': '=', 'V_u': df['V']})
      )
 .assign(TV=lambda d: d.groupby('M')['V_u'].apply(lambda g: M_dic[g.name](g) if g.name in M_dic else g))
 .groupby(['S','M'], as_index = False)
 .agg(**{'ID': ('ID', lambda x: ','.join(x.unique())),
         'V_mean': ('V_u', 'mean'),
         'V_mean_c': ('V_c', majority), ## FIXME
         'V_N': ('V_u', 'count'),
         'V_sample_sd': ('V_u', 'std'),
         'TV_mean': ('TV', 'mean'),
 })
 .assign(TV_mean=lambda d: d['V_mean_c'].mask(d['V_mean_c'].eq('='), '')+d['TV_mean'].astype(str))
)

Result:

# 52.3 s ± 436 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

---

EDIT 3 trying again, after simulating a dataset with duplicated {S, M} entries.

rng = default_rng(12345)

S = rng.choice(range(200000), 225000)
ID = S.copy()
S = [f"S_{Si}" for Si in S]
ID = [f"ID_{IDi}" for IDi in ID]
pM = [np.sqrt(1+i) for i in range(100)]
pM = pM / np.sum(pM)
M = rng.choice(range(100), 225000, p = pM)
M_to_transf_dict = dict()
for i in range(0,10):
    M_to_transf_dict[f"M_{i}"] = 'exp(x)'
for i in range(10,30):
    M_to_transf_dict[f"M_{i}"] = 'x'
for i in range(30,100):
    M_to_transf_dict[f"M_{i}"] = 'log10(x)'
M = [f"M_{Mi}" for Mi in M]
S = S + S
ID = ID + ID
M = M + M
V = rng.random(450000)
Q = rng.choice(['', '<', '>'], 450000, p = [0.9, 0.05, 0.05])
V = [f"{q}{v}" for q, v in zip(Q, V)]
df0 = pd.DataFrame({'S' : S, 'ID' : ID, 'M' : M, 'V' : V})

Original code:

10.6 s ± 154 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

@mozway's code:

25.8 s ± 550 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

(and the output looks different).

Answer 1

Your code is slow due to loops, apply and eval .

Here is a faster approach ( provided your groups are of sufficient size ):

def majority(s):
    mode = s.mode()
    return '=' if len(mode)>1 else mode.iloc[0]

M_dic = {'Y' : np.exp, 'Z' : np.log10}

(df
 .join(df['V']
       .str.extract('(?P<V_c>\D)?(?P<V_u>\d+(?:\.\d+)?)')
       .astype({'V_u': float}).fillna({'V_c': '=', 'V_u': df['V']})
      )
 .assign(TV=lambda d: d.groupby('M')['V_u'].apply(lambda g: M_dic[g.name](g) if g.name in M_dic else g))
 .groupby(['S','M'], as_index = False)
 .agg(**{'ID': ('ID', lambda x: ','.join(x.unique())),
         'V_mean': ('V_u', 'mean'),
         'V_mean_c': ('V_c', majority), ## FIXME
         'V_N': ('V_u', 'count'),
         'V_sample_sd': ('V_u', 'std'),
         'TV_mean': ('TV', 'mean'),
 })
 .assign(TV_mean=lambda d: d['V_mean_c'].mask(d['V_mean_c'].eq('='), '')+d['TV_mean'].astype(str))
)

output:

   S  M       ID     V_mean V_mean_c  V_N  V_sample_sd              TV_mean
0  A  X  ID1,ID2   3.166667        =    3     2.753785   3.1666666666666665
1  B  Y      ID4  10.000000        >    1          NaN  >22026.465794806718
2  B  Z      ID4   7.000000        =    1          NaN   0.8450980400142568
3  C  Y      ID3   1.000000        <    1          NaN   <2.718281828459045