设置一个空的 pandas 数据框的列 [英] Setting columns for an empty pandas dataframe

问题描述

这是我很困惑的事情...

This is something that I'm confused about...

import pandas as pd

# this works fine
df1 = pd.DataFrame(columns=['A','B'])

# but let's say I have this
df2 = pd.DataFrame([])

# this doesn't work!
df2.columns = ['A','B']
# ValueError: Length mismatch: Expected axis has 0 elements, new values have 2 elements

为什么这行不通?我该怎么办呢?是做这样的事情的唯一方法吗?

Why doesn't this work? What can I do instead? Is the only way to do something like this?

if len(df2.index) == 0:
    df2 = pd.DataFrame(columns=['A','B'])
else:
    df2.columns = ['A','B']

必须有一种更优雅的方式.

There must be a more elegant way.

谢谢您的帮助！

有人问为什么要这么做:

Someone asked why do this at all:

df2 = pd.DataFrame([])

原因是实际上我在做这样的事情:

The reason is that actually I'm doing something like this:

df2 = pd.DataFrame(data)

...其中数据可以是列表的空列表，但在大多数情况下不是.所以是的，我可以做:

... where data could be empty list of lists, but in most cases it is not. So yes, I could do:

if len(data) > 0:
    df2 = pd.DataFrame(data, columns=['A','B'])
else:
    df2 = pd.DataFrame(columns=['A','B'])

...但是看起来不太干(当然也不太简洁).

... but this doesn't seem very DRY (and certainly not concise).

如果您有任何疑问，请告诉我.谢谢！

Let me know if you have any questions. Thanks!

推荐答案

这看起来像是熊猫中的错误.所有这些工作:

This looks like a bug in pandas. All of these work:

pd.DataFrame(columns=['A', 'B'])
pd.DataFrame({}, columns=['A', 'B'])
pd.DataFrame(None, columns=['A', 'B'])

但不是这样:

pd.DataFrame([], columns=['A', 'B'])

在修复之前，我建议类似以下内容:

Until it's fixed, I suggest something like this:

if len(data) == 0: data = None
df2 = pd.DataFrame(data, columns=['A','B'])

或:

df2 = pd.DataFrame(data if len(data) > 0 else None, columns=['A', 'B'])

这篇关于设置一个空的 pandas 数据框的列的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！