Python + 正则表达式：如何在Python中提取两个下划线之间的值？

我正在尝试提取两个下划线之间的值。为此我写了这段代码：

patient_ids = []
for file in files:
    print(file)
    patient_id = re.findall("_(.*?)_", file)
    patient_ids.append(patient_id)

print(patient_ids) 

Output:

PT_112_NIM 26-04-2017_merged.csv
PT_114_NIM_merged.csv
PT_115_NIM_merged.csv
PT_116_NIM_merged.csv
PT_117_NIM_merged.csv
PT_118_NIM_merged.csv
PT_119_NIM_merged.csv
[['112'], ['114'], ['115'], ['116'], ['117'], ['118'], ['119'], ['120'], ['121'], ['122'], ['123'], ['124'], ['125'], ['126'], ['127'], ['128'], ['129'], ['130'], ['131'], ['132'], ['133'], ['134'], ['135'], ['136'], ['137'], ['138'], ['139'], ['140'], ['141'], ['142'], ['143'], ['144'], ['145'], ['146'], ['147'], ['150'], ['151'], ['152'], ['153'], ['154'], ['155'], ['156'], ['157'], ['158'], ['159'], ['160'], ['161'], ['162'], ['163'], ['165']]

因此提取的值采用以下形式：['121']。我希望它们采用这种形式： 121 ，即只是两个下划线内的数字。

我应该对我的代码进行哪些更改？

实际上，一个简单的方法是，而不是附加list到另一个list，只需使该列表等效：

patient_ids = []
for file in files:
    print(file)
    patient_ids.extend(re.findall("_(.*?)_", file))

print(patient_ids)