Exec in function write unwanted variables in workspace - dataframe

Question

Problem description:
I have written a code to load files from an folder into a function which puts tdms files into one single dataframe. After putting this code into a function problems appeared. I know the root of the problem is around defining the variables in the scope. I would like my function to only output "dataFrame". Instead the global in the exec function leads to the dataFrame_1,2,... in the workspace. How can I avoid this from happening?
My code in a function:
#%% Import
import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
import os, glob, sqlite3
import tkinter as tk
from tkinter import filedialog
from nptdms import TdmsFile
#%% Load data
def get_dataframe():
"""
The function takes a folder path with a path dialogue and put all
tdms-files in one dataframe.
Returns
-------
TYPE
Dataframe.
"""
# select folder to load data from
def select_folder():
root = tk.Tk()
root.attributes('-topmost',True)
root.withdraw()
print("Please select a folder with the tdms-files inside...")
folder_root = filedialog.askdirectory()
return folder_root
folder = select_folder()
os.chdir(folder)
# put data in one dataframe
i = 1
df_list = []
for path, subdirs, files in os.walk(folder):
for file in files:
if file.endswith(".tdms"):
# print(os.path.join(path, file))
os.chdir(path)
exec("global tdms_file%d; tdms_file%d = TdmsFile.read(file)"
% (i,i))
exec("tdms_file%d.close()" % (i))
exec("global dataFrame_%d; global tdms_file%d; \
dataFrame_%d = tdms_file%d.\
as_dataframe(time_index=True)" % (i,i,i,i))
exec("global tdms_file%d; del tdms_file%d" % (i,i))
df_list.append("dataFrame_%d" % (i))
i += 1
dataFrame = pd.concat([eval(element) for element in df_list], axis=1)
Burst_name = ["Burst {0}".format(i) for i in range(dataFrame.shape[1])]
dataFrame.columns = Burst_name
return dataFrame
dataFrame = get_dataframe()
Outside of the function this part works fine:
# put data in one dataframe
i = 1
df_list = []
for path, subdirs, files in os.walk(folder):
for file in files:
if file.endswith(".tdms"):
# print(os.path.join(path, file))
os.chdir(path)
exec("tdms_file%d = TdmsFile.read(file)" % (i))
exec("tdms_file%d.close()" % (i))
exec("dataFrame_%d = tdms_file%d.as_dataframe(time_index=True)" % (i,i))
exec("del tdms_file%d" % (i,i))
df_list.append("dataFrame_%d" % (i))
i += 1