baseline functions differ in R and Python

Hello! I am translating the Cox model from R to Python and found that baseline functions in R differs from the baseline functions in Python. Based on the data from the file test.xlsx, the results are as follows:

It seemed strange to me, considering that the lifelines library is based on the codes of functions from R (if I understand correctly).

At the same time, the characteristics of the Cox model and the coefficient before regressor “var_const” turned out to be the same in R and Python (coef[var_cost] = 0,05295).

Tell me please why baseline functions may differ in R and Python?

Code in R:

library("survival")
library(survminer)

#### Data download:
test_R <- read_excel("test.xlsx")

#### Data preparation:
test_R$start = test_R$months
test_R$stop = test_R$start + 1

#### Run the model:
res.cox1 <- coxph(Surv(start, stop, event)~ var_const, data = test_R, method = 'efron')
summary(res.cox1)

#### Calculate baseline cumulative hazard function:
bhest = basehaz(fit = res.cox1)
basehaz_predict <- data.frame(time = bhest$time,
                              hazard = bhest$hazard)

#### Calculate baseline survival function
cox_survival = survfit(res.cox1)
surv_predict <- data.frame(time = cox_survival$time,
                           survival = cox_survival$surv)

Code in Python:

import pandas as pd
import numpy as np
from lifelines.utils import to_long_format
from lifelines import CoxTimeVaryingFitter

#### Data download:
df = pd.read_excel('test.xlsx')

#### Data preparation:
df_model = df_model[['id', 'event', 'months', ‘var_const’]]
df_model = to_long_format(df_model, duration_col = "months")
df_model['start'] = df_model['stop']
df_model['stop'] = df_model['start']+1

#### Run the model:
ctv = CoxTimeVaryingFitter()
ctv.fit(df_model, id_col ='id', event_col='event', start_col = 'start', stop_col = 'stop')
ctv.print_summary()

#### Calculate baseline cumulative hazard function:
ctv.baseline_cumulative_hazard_

#### Calculate baseline survival function:
ctv.baseline_survival_

[Valery2511]

@CamDavidsonPilon, may be you will be interested, I also apply a file with the results of comparison of the baseline functions not only of R and Python but also of STATA (the dataset is the same - test.xlsx): Compare_baseline_functions_R_Python_STATA.xlsx

The most interesting thing is that all three programs give three slightly different results:)

Code in STATA:

#### Data download:
import excel "test.xlsx", sheet("Sheet1") firstrow
sort id months

#### Run the model:
stset months, id(id) failure(event)
stcox var_const, efron

#### Calculate baseline cumulative hazard function:
predict S0_hazard_cumulative, basechazard

#### Calculate baseline survival function:
predict S0_baseline_survivor, basesurv  

[PortlandMichelle]

Hello - TBH I discovered the difference using a data set that I would not be able to share (due to confidentiality concerns), so I replicated it with the data provided in this post by the original poster. I find the identical results they did (in their original screen shot at the top of this post). That’s why I was hoping there’d been some analysis using that data set that helped illuminate the issue back in 2020. But if you’re able to do some investigation now, if we could both use the original data when this question was originally made, that would be great.