This seems to be the same issue as #4584 (https://github.com/pandas-dev/pandas/issues/4584), but the issue seems to persist.

I'm using Pandas 0.19.1. The docs state that for to_hdf(), "mode = 'a' " means "Append; an existing file is opened for reading and writing" http://pandas.pydata.org/pandas-docs/version/0.19.1/generated/pandas.DataFrame.to_hdf.html

But the following code example shows to_hdf(... mode='a') is overwriting, not appending 

df1 = pd.DataFrame(np.random.randint(0,100,size=(100, 4)), columns=list('ABCD'))
df2 = pd.DataFrame(np.random.randint(0,100,size=(100, 4)), columns=list('ABCD'))
df3 = pd.DataFrame(np.random.randint(0,100,size=(100, 4)), columns=list('ABCD'))
df4 = pd.DataFrame(np.random.randint(0,100,size=(100, 4)), columns=list('ABCD'))

for df in (df1, df2, df3, df4):
    df.to_hdf('example.h5', 'my_hdf_table', mode='a', format='table')    

store = pd.HDFStore('example.h5')
print(store.items)    
#prints "/my_hdf_table             frame_table  (typ->appendable,nrows->100,..."

However, using df.to_hdf('example.h5', 'my_hdf_table2', format='table', append=True) does result in appending, not overwriting.

Maybe this is a documentation issue not a code issue? Why do we have "mode = a" described as "append" if a separate non-default option "append = True" is still required for appending?

As always, thanks for all the work on a fantastic library.

Comment From: jreback

you misunderstand

mode is about the file itself

a node has possibilities of different formats (e.g. table can be appended while fixed cannot)

if you think that the documentation is unclear please submit a PR for a change

Comment From: AlyShmahell

Kudos for noticing @MaxPowerWasTaken
And it's true what @jreback said, at the moment, the doc seems to be clear:

mode : {‘a’, ‘w’, ‘r+’}, default ‘a’
    Mode to open file:
         ‘w’: write, a new file is created (an existing file with the same name would be deleted).
         ‘a’: append, an existing file is opened for reading and writing, and if the file does not exist it is created.
         ‘r+’: similar to ‘a’, but the file must already exist.
append : bool, default False
    For Table formats, append the input data to the existing.