pandas-alchemy

pandas-alchemy is a Python package for analyzing data in SQL databases using a pandas compatible interface.

While SQL the language absolutely sucks, SQL databases are great. Databases shine at handling larger-than-memory data. There have been countless hours of engineering spent on optimizing their performance. Furthermore, often times, the data we need to analyze is already in some databases.

pandas-alchemy implements SQL based DataFrame and Series. Unlike read_sql() in pandas, the entire table is not loaded into the main memory. Instead, a SQL query is stored and is used to back a DataFrame or a Series.

import pandas as pd
from pandas_alchemy import init_db, DataFrame


# Connect to the database using an sqlalchemy database URL
init_db('postgresql://foo:bar@localhost/quux')
df = DataFrame.from_table('foobar')
df.head()  # Just treat it like a pandas DataFrame
df + [1,2,3]  # Arithmetic works too

pd_df = pd.DataFrame({'baz': [1, 2], 'quux': ['a', 'b']})
df + pd_df  # pandas interoperable

# Getting a Series from the DataFrame
# Data is not actually loaded into memory, either
df.baz

# Actually retrieve data and convert to a pandas DataFrame
df.to_pandas()

:warning: Unusability Warning :warning:

This project is in its very early development. Many features are still missing. While it is the goal of this project, it is not possible, as of now, to replace a pandas DataFrame or Series with a pandas-alchemy counterpart and to expect the program to JustWork^:tm:.

pandas_alchemy.use_repr_workaround()

Currently, DataFrame.__repr__() and Series.__repr__() is not implemented yet. By calling pandas_alchemy.use_repr_workaround(), DataFrame.__repr__() and Series.__repr__() will retrieve all data, convert itself to a pandas counterpart, and then repr() that pandas counterpart. Note that this will usually defeat the whole point of using pandas-alchemy.

Exception-to-exception compatibility

pandas-alchemy aims to be completely transparent to the program. There should be zero difference between the behaviour of a pandas-alchemy DataFrame and a pandas DataFrame, even when an exception is raised. Differences between behaviours, including differences in exceptions raised (to a reasonable extent), are considered bugs for the purpose of this project. Please open an issue if you have spotted such a difference.

Usage

1. pandas_alchemy.init_db(<sqlalchemy database URL>)
2. df = pandas_alchemy.DataFrame.from_table(<table>)
3. Treat df as a pandas DataFrame and do something awesome about it.
4. Profit!!!

init_db(*args, **kwargs)

Create a database connection. If already connected, raise RuntimeError.

*args and **kwargs are passed directly to sqlalchemy.create_engine(). See the documentation of sqlalchemy.create_engien()

close_db()

Close the database connection. If not connected yet, raise RuntimeError.

DataFrame(index, columns, cte)

Probably not something you are looking for.

Create a DataFrame using index, columns and cte. index is a pandas Index representing the levels in the DataFrame's index. columns is a pandas Index representing the column names. cte is a sqlalchemy CTE that backs the DataFrame.

DataFrame.from_table(table, schema=None, columns=None, index=None)

Load table from the database as a DataFrame.

If columns is not None, it is taken as an ordered list of columns to be included in the DataFrame.

If index is a list-like object, it is taken as an ordered list of columns whose values are the DataFrame's index. Otherwise, if index is not None, it is taken as the name of the column to become the DataFrame's index.

DataFrame.from_pandas(df, optional=False)

Convert the pandas DataFrame df to a DataFrame. If df is not an instance of pandas.DataFrame, return it as is when optional is True. Otherwise, raise TypeError.

DataFrame.to_pandas()

Convert the DataFrame to a pandas DataFrame.

Series(index, columns, cte, name)

Probably not something you are looking for.

Create a Series using index, columns and cte, with name name. index is a pandas Index representing the levels in the Series's index. columns is a pandas Index representing the column names. cte is a sqlalchemy CTE that backs the Series. name is the name of the Series.

Series.from_pandas(seq, name=None, optional=False)

Convert the pandas Series seq to a Series. If name is not None, it will be used as the name of the resulting Series instead of seq.name. If seq is not an instance of pandas.Series, return it as is when optional is True, Otherwise, raise TypeError.

Series.to_pandas()

Convert the Series to a pandas Series.

pandas API Coverage

See API_COVERAGE.md.

Known Issues & Limitations

• Cannot distinguish 0.0 and -0.0 (IEEE float)
• Returns None for NaN in SQLite3 if every value in the column is None
• Lacks support for arithmetic between two MultiIndex DataFrame/Series