DataFrame.ix() in pandas - is there an option to catch situations when requested columns do not exist? - dataframe

Question

My code reads CSV file into pandas DataFrame - and processes it.
The code relies on column names - uses df.ix[,] to get the columns.
Recently some column names in the CSV file were changed (without notice).
But the code was not complaining and was silently producing wrong results.
The ix[,] construct doesn't check if column exists.
If it doesn't - it simply creates it and populate with NaN.
Here is the main idea of what was going on.
df1=DataFrame({'a':[1,2,3],'b':[4,5,6]}) # columns 'a' & 'b'
df2=df1.ix[:,['a','c']] # trying to get 'a' & 'c'
print df2
a c
0 1 NaN
1 2 NaN
2 3 NaN
So it doesn't produce an error or a warning.
Is there an alternative way to select specific columns with extra check that columns exist?
My current workaround is to use my own small utility function, something like this:
import sys, inspect
def validate_cols_or_exit(df,cols):
"""
Exits with error message if pandas DataFrame object df
doesn't have all columns from the provided list of columns
Example of usage:
validate_cols_or_exit(mydf,['col1','col2'])
"""
dfcols = list(df.columns)
valid_flag = True
for c in cols:
if c not in dfcols:
print "Error, non-existent DataFrame column found - ",c
valid_flag = False
if not valid_flag:
print "Error, non-existent DataFrame column(s) found in function ", inspect.stack()[1][3]
print "valid column names are:"
print "\n".join(df.columns)
sys.exit(1)

Answer 1

How about:
In [3]: df1[['a', 'c']]
---------------------------------------------------------------------------
KeyError Traceback (most recent call last)
/home/wesm/code/pandas/<ipython-input-3-2349e89f1bb5> in <module>()
----> 1 df1[['a', 'c']]
/home/wesm/code/pandas/pandas/core/frame.py in __getitem__(self, key)
1582 if com._is_bool_indexer(key):
1583 key = np.asarray(key, dtype=bool)
-> 1584 return self._getitem_array(key)
1585 elif isinstance(self.columns, MultiIndex):
1586 return self._getitem_multilevel(key)
/home/wesm/code/pandas/pandas/core/frame.py in _getitem_array(self, key)
1609 mask = indexer == -1
1610 if mask.any():
-> 1611 raise KeyError("No column(s) named: %s" % str(key[mask]))
1612 result = self.reindex(columns=key)
1613 if result.columns.name is None:
KeyError: 'No column(s) named: [c]'

Answer 2

Not sure you can constrain a DataFrame, but your helper function could be a lot simpler. (something like)
mismatch = set(cols).difference(set(dfcols))
if mismatch:
raise SystemExit('Unknown column(s): {}'.format(','.join(mismatch)))