编辑: 这个问题不是pandas dataframe replace nan values with average of columns的克隆，因为我想用列的平均值替换每一列的值，而不是用数据框中所有值的平均值。
问题
我有一个带有一百列的pandas数据框（train），需要应用机器学习技术。
通常情况下，我手工进行特征工程，但在这种情况下，我有很多列要处理。
我想构建一个Python函数：
1) 找到每一列中的NaN值（我考虑使用df.isnull().any()）
2) 对于每个NaN值，将其替换为找到NaN值的列的平均值。
我的想法是这样的：
def replace(value):
for value in train:
if train['value'].isnull():
train['value'] = train['value'].fillna(train['value'].mean())
train = train.apply(replace,axis=1)
但是我收到了以下错误信息
---------------------------------------------------------------------------
KeyError Traceback (most recent call last)
/opt/conda/lib/python3.6/site-packages/pandas/core/indexes/base.py in get_loc(self, key, method, tolerance)
3063 try:
-> 3064 return self._engine.get_loc(key)
3065 except KeyError:
pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()
pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()
pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item()
pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item()
KeyError: 'value'
During handling of the above exception, another exception occurred:
KeyError Traceback (most recent call last)
<ipython-input-25-003b3eb2463c> in <module>()
----> 1 train = train.apply(replace,axis=1)
/opt/conda/lib/python3.6/site-packages/pandas/core/frame.py in apply(self, func, axis, broadcast, raw, reduce, result_type, args, **kwds)
6012 args=args,
6013 kwds=kwds)
-> 6014 return op.get_result()
6015
6016 def applymap(self, func):
/opt/conda/lib/python3.6/site-packages/pandas/core/apply.py in get_result(self)
140 return self.apply_raw()
141
--> 142 return self.apply_standard()
143
144 def apply_empty_result(self):
/opt/conda/lib/python3.6/site-packages/pandas/core/apply.py in apply_standard(self)
246
247 # compute the result using the series generator
--> 248 self.apply_series_generator()
249
250 # wrap results
/opt/conda/lib/python3.6/site-packages/pandas/core/apply.py in apply_series_generator(self)
275 try:
276 for i, v in enumerate(series_gen):
--> 277 results[i] = self.f(v)
278 keys.append(v.name)
279 except Exception as e:
<ipython-input-22-2e7fa654e765> in replace(value)
1 def replace(value):
2 for value in train:
----> 3 if train['value'].isnull():
4 train['value'] = train['value'].fillna(df['value'].mean())
/opt/conda/lib/python3.6/site-packages/pandas/core/frame.py in __getitem__(self, key)
2686 return self._getitem_multilevel(key)
2687 else:
-> 2688 return self._getitem_column(key)
2689
2690 def _getitem_column(self, key):
/opt/conda/lib/python3.6/site-packages/pandas/core/frame.py in _getitem_column(self, key)
2693 # get column
2694 if self.columns.is_unique:
-> 2695 return self._get_item_cache(key)
2696
2697 # duplicate columns & possible reduce dimensionality
/opt/conda/lib/python3.6/site-packages/pandas/core/generic.py in _get_item_cache(self, item)
2484 res = cache.get(item)
2485 if res is None:
-> 2486 values = self._data.get(item)
2487 res = self._box_item_values(item, values)
2488 cache[item] = res
/opt/conda/lib/python3.6/site-packages/pandas/core/internals.py in get(self, item, fastpath)
4113
4114 if not isna(item):
-> 4115 loc = self.items.get_loc(item)
4116 else:
4117 indexer = np.arange(len(self.items))[isna(self.items)]
/opt/conda/lib/python3.6/site-packages/pandas/core/indexes/base.py in get_loc(self, key, method, tolerance)
3064 return self._engine.get_loc(key)
3065 except KeyError:
-> 3066 return self._engine.get_loc(self._maybe_cast_indexer(key))
3067
3068 indexer = self.get_indexer([key], method=method, tolerance=tolerance)
pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()
pandas/_libs/index.pyx in pandas._libs.index.IndexEngine.get_loc()
pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item()
pandas/_libs/hashtable_class_helper.pxi in pandas._libs.hashtable.PyObjectHashTable.get_item()
KeyError: ('value', 'occurred at index 0')
在寻找解决方案时，我发现：