my_str = "地址"
my_df = pd.DataFrame([["序号0", True], ["序号1", False]], columns=["序号", "判断"])

# 遍历判断,拼接字符串(不能使用函数拼接,譬如(os.path.join或str.join), 除非指定到行)
my_df["拼接"] = np.where(
    my_df.判断,
    my_str + "\\" + my_df.序号,
    "不拼接"
)
my_df

# 遍历判断,匹配字典,拼接字符串(遍历内部不能使用函数调用表格,除非映射到相应的行)
# 如果my_df["判断"]为True,将my_str与my_dict中对应的键值按"\\"拼接
my_dict = {"序号0": "文件0", "序号3": "文件1"}
my_df["匹配"] = np.where(
    my_df.判断,
    my_str + "\\" + my_df["序号"].map(my_dict),  # 将键值映射到相应的行
    "不拼接"
)
my_df

import pandas as pd
import numpy as np
import os

my_str = "地址"
my_df = pd.DataFrame([["序号0", True], ["序号1", False]], columns=["序号", "判断"])
my_df["拼接"] = np.where(
    my_df["判断"],
    np.vectorize(os.path.join)(my_str, my_df["序号"]),
    "不拼接"
)
my_df

import pandas as pd
import numpy as np
import os

my_str = "地址"
my_df = pd.DataFrame([["序号0", True], ["序号1", False]], columns=["序号", "判断"])
my_df["拼接"] = np.where(
    my_df["判断"],
    np.vectorize(lambda x: "\\".join(x))([my_str, my_df["序号"]]),
    "不拼接"
)
my_df