I tried coding like this:

input_filter = {"$and": []}

if not includeTestAccount:
    b = {"userLoginKey": {"$not": {"$regex": "^777"}}}
    input_filter["$and"].append(b)

if search_user_login_key is not None and search_user_login_key != "":
    b = {"userLoginKey":{"$regex":search_user_login_key}}
    input_filter["$and"].append(b)

if except_for_deletedaccount:
    b = {"deletedAt": {"$not": {"$regex": None}}}
    input_filter["$and"].append(b)

if except_for_deletedaccount:
    b = {"deletedAt": {"$not": {"$regex": None}}}
    input_filter["$and"].append(b)

but there was error in pymongo?:

pymongo.errors.OperationFailure: $and/$or/$nor must be a nonempty array,
full error: {'ok': 0.0, 'errmsg': '$and/$or/$nor must be a nonempty array', 'code': 2, 'codeName': 'BadValue'}