「TXTを1つずつ読み込んで空のDataframeに結合」を繰り返す [Python][Pandas]

from pathlib import Path

def txt_to_df(str_txt):
    # TXTをDataframeに変換する、何らかの処理
    return df

txt_path = Path("./txt/")
all_df = pd.DataFrame()

for txt in txt_path.glob("*.txt"):

    str_txt = txt.read_text()

    df = txt_to_df(str_txt)

    all_df = pd.concat()