Replacing a value with actual if condition with DataFrame in Python

I need help with implementing actual if conditions with dataframe. I know the replace and where functions, but unfortunately I am not directly able to use them.

Basically I want to change value of a column based on whether a folder exists with a UID. If the folder exists in related directory I want to place yes, if not, I want to place no. For example: if D:\data\folder\00010002 exists, then folder value of the UID should be yes. Else, it should be no. How can I do that?

import os
import pandas as pd

d = {'UID': ["00010002", "00010004"], 'folder': ['a', 'a']}
df = pd.DataFrame(data=d)

for a in d['UID']:
    if os.path.isdir(f'D:\data\folder\{a}'):
        df.loc[df.folder] == 'yes'
    else:
        df.loc[df.folder] == 'no'

>Solution :

Try this:

df['folder'] = df['UID'].apply(lambda uid: 'yes' if os.path.isdir(f'D:\\data\\folder\\{uid}') else 'no')