python - why filter function in python3 always returns empty result?

Question

I am learning generator in python.

data = read_fake_data("./data/fake_data.csv") # genrator function
data_day = day_grouper(data) # generator function, group data by key
print("data grouped:",list(data_day)) 
# it prints:
# data grouped: [('2019-01-01', <itertools._grouper object at 0x000002294D296EB8>)]
# work well

then I want function #check_anomaly# to filter anomalous data:

ddd = filter(check_anomaly, data_day)
print("data filtered:",list(ddd))

only find that it prints empty list in console I suspect error may occure in function , so I try to keep all data to debug:

ddd = filter(lambda x: x, data_day)
print("data filtered:",list(ddd))

It still returns empty result!

data filtered: []

Why is this happening? Can anybody help me with this???

标签： pythonfiltergenerator