Why does pandas roll a week forward when using resample with W-MON frequency?

Question

As example, I have the following code which creates a dataframe with an index containing a single value - the date '2018-03-06' (a Tuesday). Note that this date falls in the week of 2018-03-05 (a Monday):

values = [1, 1, 1]
dates = pd.to_datetime(np.repeat('2018-03-06', 3))
df = pd.DataFrame({
    'value': values
}, index=dates)
df.resample('W-MON').size()

which produces:

2018-03-12    3
Freq: W-MON, dtype: int64

Why does pandas roll the date forward one week? I would have expected the result to have been resampled to 2018-03-05 since that is the week during which the values were generated and I'm using freq='W-MON'.

UPDATE

As was pointed out, I needed to add the label argument to resample which defines which bin edge to use. Using label='left' solves the problem of bucketing the dates in the correct week except when the date falls on the start of the week (in this case, Monday). For example, if I apply resample to the date 2018-03-05 using label='left' then the resampled value is 2018-02-26 when it should be 2018-03-05.

Answer 1

Let's try using label and closed see docs:

values = [1, 1, 1]
dates = pd.to_datetime(np.repeat('2018-03-06', 3))
df = pd.DataFrame({
    'value': values
}, index=dates)
df.resample('W-MON', label='left',closed='left').size()

Output:

2018-03-05    3
Freq: W-MON, dtype: int64

And,

values = [1, 1, 1]
dates = pd.to_datetime(np.repeat('2018-03-05', 3))
df = pd.DataFrame({
    'value': values
}, index=dates)
df.resample('W-MON', label='left',closed='left').size()

Output:

2018-03-05    3
Freq: W-MON, dtype: int64

Interesting note about the docs, the signature states that 'closed' defaults to None. However, the docstring states that 'closed' default 'left'.

Answer 2

I'm not sure why it's done this way and I agree that the behaviour you expected seems more intuitive. You can get your desired result by passing label='left' as a keyword parameter. The default value in this case was 'right'.

df.resample('W-MON', label='left').size()

From the documentation:

label : {‘right’, ‘left’}

Which bin edge label to label bucket with. The default is ‘left’ for all frequency offsets except for ‘M’, ‘A’, ‘Q’, ‘BM’, ‘BA’, ‘BQ’, and ‘W’ which all have a default of ‘right’.

I guess 'W-MON' still counts as 'W' which is why the default is 'right' and therefore your example gave a result of '2018-03-12' rather than '2018-03-05'.