I need convert the data stored in a pandas.DataFrame
into a byte string where each column can have a separate data type (integer or floating point). Here is a simple set of data:
df = pd.DataFrame([ 10, 15, 20], dtype='u1', columns=['a']) df['b'] = np.array([np.iinfo('u8').max, 230498234019, 32094812309], dtype='u8') df['c'] = np.array([1.324e10, 3.14159, 234.1341], dtype='f8')
and df looks something like this:
a b c 0 10 18446744073709551615 1.324000e+10 1 15 230498234019 3.141590e+00 2 20 32094812309 2.341341e+02
The DataFrame
knows about the types of each column df.dtypes
so I’d like to do something like this:
data_to_pack = [tuple(record) for _, record in df.iterrows()] data_array = np.array(data_to_pack, dtype=zip(df.columns, df.dtypes)) data_bytes = data_array.tostring()
This typically works fine but in this case (due to the maximum value stored in df['b'][0]
. The second line above converting the array of tuples to an np.array
with a given set of types causes the following error:
OverflowError: Python int too large to convert to C long
The error results (I believe) in the first line which extracts the record as a Series
with a single data type (defaults to float64
) and the representation chosen in float64
for the maximum uint64
value is not directly convertible back to uint64
.
1) Since the DataFrame
already knows the types of each column is there a way to get around creating a row of tuples for input into the typed numpy.array
constructor? Or is there a better way than outlined above to preserve the type information in such a conversion?
2) Is there a way to go directly from DataFrame
to a byte string representing the data using the type information for each column.
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Answer
You can use df.to_records()
to convert your dataframe to a numpy recarray, then call .tostring()
to convert this to a string of bytes:
rec = df.to_records(index=False) print(repr(rec)) # rec.array([(10, 18446744073709551615, 13240000000.0), (15, 230498234019, 3.14159), # (20, 32094812309, 234.1341)], # dtype=[('a', '|u1'), ('b', '<u8'), ('c', '<f8')]) s = rec.tostring() rec2 = np.fromstring(s, rec.dtype) print(np.all(rec2 == rec)) # True