Fill new column based on conditions defined in a string

You can use np.where:

df['results'] =  np.where((((df['colA']=='yes') & (df['colB']=='yes')) & ((df['colC']=='yes') | (df['colD']=='yes'))), 'Yes',np.where(((df['colA']=='no') & (df['colB']=='no')) & ((df['colC']=='no' )| (df['colD']=='no')), 'No','UNKNOWN'))

which gives:

 ID colA colB colC colD decision
0  AB01  yes  NaN  yes  yes  UNKNOWN
1  AB02  yes  yes  yes   no      Yes
2  AB03  yes  yes  yes  yes      Yes
3  AB03   no   no   no   no       No
4  AB04   no   no   no  NaN       No
5  AB05  yes  NaN  NaN   no  UNKNOWN
6  AB06  NaN  yes  NaN  NaN  UNKNOWN

IIUC you want to create arbitrary conditions for your df, which can be done using functools.reduce and operator.and_. You can then set up your conditions using two lists (instead of a dict), first one being the columns, second one the string to test against and finally np.select:

from functools import reduce
from operator import and_

cols = ["colA", "colB", ["colC", "colD"]] # group the cols in a list if they belong to the same group
answer = ["yes", "no"]

conds = [reduce(and_, [df[i].eq(ans) if isinstance(i, str) else df[i].eq(ans).any(1)
                       for i in cols]) for ans in answer]

df["result"] = np.select(conds, answer, "Unknown")

print (df)

     ID colA colB colC colD   result
0  AB01  yes  NaN  yes  yes  Unknown
1  AB02  yes  yes  yes   no      yes
2  AB03  yes  yes  yes  yes      yes
3  AB03   no   no   no   no       no
4  AB04   no   no   no  NaN       no
5  AB05  yes  NaN  NaN   no  Unknown
6  AB06  NaN  yes  NaN  NaN  Unknown

Now you simply need to edit the two lists cols and answer if you need to adjust your conditions.

Here a solution to convert your condition to a python function and then applying it to the rows of your DataFrame:

import re

condition_string =  "colA='yes' & colB='yes' & (colC='yes' | colD='yes'): 'Yes', colA='no' & colB='no' & (colC='no' | colD='no'): 'No', ELSE : 'UNKNOWN'"

# formatting string as python function apply_cond
for col in df.columns:
    condition_string = re.sub(rf"(\W|^){col}(\W|$)", rf"\1row['{col}']\2", condition_string)
    condition_string = re.sub(rf"row\['{col}'\]\s*=(?!=)", f"row['{col}']==", condition_string)

cond_form = re.sub(r'(:[^[(]+), (?!ELSE)', r'\1\n\telif ', condition_string) \
            .replace(": ", ":\n\t\treturn ") \
            .replace("&", "and") \
            .replace('|', 'or')
cond_form = re.sub(r", ELSE\s*:", "\n\telse:", cond_form)
function_def = "def apply_cond(row):\n\tif " + cond_form
#print(function_def) # uncomment to see how the function is defined

# executing the function definition of apply_cond
exec(function_def)

# applying the function to each row
df["result"]=df.apply(lambda x: apply_cond(x), axis=1)

print(df)

Output:

     ID colA colB colC colD   result
0  AB01  yes  NaN  yes  yes  UNKNOWN
1  AB02  yes  yes  yes   no      Yes
2  AB03  yes  yes  yes  yes      Yes
3  AB03   no   no   no   no       No
4  AB04   no   no   no  NaN       No
5  AB05  yes  NaN  NaN   no  UNKNOWN
6  AB06  NaN  yes  NaN  NaN  UNKNOWN

You might want to adapt string formatting depending on condition_string (I did it quickly, there might be some unsupported combinations) but if you get those strings automatically it will save you from defining them all over again.