pandas get column average/mean

I can’t get the average or mean of a column in pandas. A have a dataframe. Neither of things I tried below gives me the average of the column weight

>>> allDF 
         ID           birthyear  weight
0        619040       1962       0.1231231
1        600161       1963       0.981742
2      25602033       1963       1.3123124     
3        624870       1987       0.94212

The following returns several values, not one:

allDF[['weight']].mean(axis=1)

So does this:

allDF.groupby('weight').mean()