Adding a string to all keys in dictionary (Python)

Try a dictionary comprehension:

{k+'@d1': v for k, v in d.items()}

In Python 3.6+, you can use f-strings:

{f'{k}@d1': v for k, v in d.items()}

You can use dict constructor to rebuild the dict, appending file number to the end of each key:

>>> d = {'a': 1, 'b': 2}
>>> file_number = 1
>>> dict(("{}@{}".format(k,file_number),v) for k,v in d.items())
>>> {'a@1': 1, 'b@1': 2}