# On using pytables; how do I...

A lot of this info is in [the docs for Tables](https://www.pytables.org/usersguide/libref/structured_storage.html#the-table-class).

Open the file for reading, and get a reference to a table:

```
>>> with tables.open_file(path, mode='r') as f:
...     table = f.root.some_columns.table

```

How many records are in the table?

```
>>> table.nrows
```

Search for a matching row (e.g. row matching the value `12345`)

```
>>> table = f.root.some_table.table
>>> table.read_where('some_column==12345')
array([(12345, 3928, 364544)]
    dtype=[('some_column', '<i8'), ('foo', '<i8'), ('bar', '<i8')])

>>> for row in table.where('some_column==12345')
...     print(row['some_column'], row['foo'], row['bar'])
```

Extract a single column for a matching row (similar to above):

```
>>> arr = table.read_where('some_column=12345')
>>> bar = arr[0][2]
```

Find a row for a given key (column value), and update a column in that
row of the table:

```
>>> arr = table.get_where_list('some_column==12345')
>>> key = arr[0]
>>> table.cols.bar[key] = 42
```

Construct a dataframe with all values

```
>>> arr = table.read()
>>> df = pd.DataFrame(arr)
```

Add an index to a column of the table:

```
>>> table.cols.some_column.create_index()
>>> table.reindex()  # to rebuild the index...
>>> table.reindex_dirty()  # rebuild indexes, if they are dirty.
```

How to add blosc compression? (See the [docs on Filters](https://www.pytables.org/usersguide/libref/helper_classes.html#the-filters-class))

```
>>> blosc_filter = Filters(complevel=9, complib='blosc')
>>> table = fileh.create_table( ... )
>>> table.cols.some_column.create_index(filters=blosc_filter)
```