query using string in PyTables 3 - pytables

Question

I have a table:
h5file=open_file("ex.h5", "w")
class ex(IsDescription):
A=StringCol(5, pos=0)
B=StringCol(5, pos=1)
C=StringCol(5, pos=2)
table=h5file.create_table('/', 'table', ex, "Passing string as column name")
table=h5file.root.table
rows=[
('abc', 'bcd', 'dse'),
('der', 'fre', 'swr'),
('xsd', 'weq', 'rty')
]
table.append(rows)
table.flush()
I am trying to query as per below:
find='swr'
creteria='B'
if creteria=='B':
condition='B'
else:
condition='C'
value=[x['A'] for x in table.where("""condition==find""")]
print(value)
It returns:
ValueError: there are no columns taking part in condition condition==find
Is there a way to use condition as a column name in above query?
Thanks in advance.

Answer 1

Yes, you can use Pytables .where() to search based on a condition. The problem is how you constructed your query for the table.where(condition). See Note about strings under Table.where() in the Pytables Users Guide:
A special care should be taken when the query condition includes string literals. ... Python 3 strings are unicode objects.
in Python 3, “condition” should be defined like this:
condition = 'col1 == b"AAAA"'
The reason is that in Python 3 “condition” implies a comparison between a string of bytes (“col1” contents) and an unicode literal (“AAAA”).
The simplest form of your query is shown below. It returns a subset of rows that match the condition. Note use of single and double quotes for string and unicode:
query_table = table.where('C=="swr"') # search in column C
I rewrote your example as best I could. See below. It shows several ways to enter the condition. I'm not smart enough to figure out how to combine your creteria and find variables into a single condition variable with string and unicode characters.
from tables import *
class ex(IsDescription):
A=StringCol(5, pos=0)
B=StringCol(5, pos=1)
C=StringCol(5, pos=2)
h5file=open_file("ex.h5", "w")
table=h5file.create_table('/', 'table', ex, "Passing string as column name")
## table=h5file.root.table
rows=[
('abc', 'bcd', 'dse'),
('der', 'fre', 'swr'),
('xsd', 'weq', 'rty')
]
table.append(rows)
table.flush()
find='swr'
query_table = table.where('C==find')
for row in query_table :
print (row)
print (row['A'], row['B'], row['C'])
value=[x['A'] for x in table.where('C == "swr"')]
print(value)
value=[x['A'] for x in table.where('C == find')]
print(value)
h5file.close()
Output shown below:
/table.row (Row), pointing to row #1
b'der' b'fre' b'swr'
[b'der']
[b'der']