pandas 仅使用列名创建空的DataFrame [英] Pandas create empty DataFrame with only column names

问题描述

我有一个动态的DataFrame，它工作正常，但是当没有数据要添加到DataFrame中时，我得到一个错误.因此，我需要一个解决方案以仅使用列名创建一个空的DataFrame.

I have a dynamic DataFrame which works fine, but when there are no data to be added into the DataFrame I get an error. And therefore I need a solution to create an empty DataFrame with only the column names.

现在我有这样的东西:

df = pd.DataFrame(columns=COLUMN_NAMES) # Note that there are now row data inserted.

PS:重要的是，列名仍应出现在DataFrame中.

PS: It is important that the column names would still appear in a DataFrame.

但是当我这样使用它时，我得到的结果是这样的:

But when I use it like this I get something like that as a result:

Index([], dtype='object')
Empty DataFrame

空数据框"部分很好！但是，除了索引以外，我还需要显示列.

The "Empty DataFrame" part is good! But instead of the Index thing I need to still display the columns.

我发现的一件重要事情:我正在使用Jinja2将此DataFrame转换为PDF，因此我在调出一种方法，首先将其输出为HTML，如下所示:

An important thing that I found out: I am converting this DataFrame to a PDF using Jinja2, so therefore I'm calling out a method to first output it to HTML like that:

df.to_html()

我认为这是列丢失的地方.

This is where the columns get lost I think.

Edit2: 通常，我遵循以下示例: http://pbpython.com/pdf-reports.html . CSS也来自链接. 这就是我将数据帧发送到PDF的步骤:

In general, I followed this example: http://pbpython.com/pdf-reports.html. The css is also from the link. That's what I do to send the dataframe to the PDF:

env = Environment(loader=FileSystemLoader('.'))
template = env.get_template("pdf_report_template.html")
template_vars = {"my_dataframe": df.to_html()}

html_out = template.render(template_vars)
HTML(string=html_out).write_pdf("my_pdf.pdf", stylesheets=["pdf_report_style.css"])

Edit3:

如果我在创建后立即打印出数据框，则会得到以下提示:

If I print out the dataframe right after creation I get the followin:

[0 rows x 9 columns]
Empty DataFrame
Columns: [column_a, column_b, column_c, column_d, 
column_e, column_f, column_g, 
column_h, column_i]
Index: []

这似乎是合理的，但是如果我打印出template_vars:

That seems reasonable, but if I print out the template_vars:

'my_dataframe': '<table border="1" class="dataframe">\n  <tbody>\n    <tr>\n      <td>Index([], dtype=\'object\')</td>\n      <td>Empty DataFrame</td>\n    </tr>\n  </tbody>\n</table>'

似乎这些列已经丢失了.

And it seems that the columns are missing already.

E4: 如果我打印出以下内容:

E4: If I print out the following:

print(df.to_html())

我已经得到以下结果:

<table border="1" class="dataframe">
  <tbody>
    <tr>
      <td>Index([], dtype='object')</td>
      <td>Empty DataFrame</td>
    </tr>
  </tbody>
</table>

推荐答案

您可以使用列名或索引创建一个空的DataFrame:

You can create an empty DataFrame with either column names or an Index:

In [4]: import pandas as pd
In [5]: df = pd.DataFrame(columns=['A','B','C','D','E','F','G'])
In [6]: df
Out[6]:
Empty DataFrame
Columns: [A, B, C, D, E, F, G]
Index: []

或

In [7]: df = pd.DataFrame(index=range(1,10))
In [8]: df
Out[8]:
Empty DataFrame
Columns: []
Index: [1, 2, 3, 4, 5, 6, 7, 8, 9]

即使您对.to_html进行了修改，我也无法复制.这个:

Even after your amendment with the .to_html, I can't reproduce. This:

df = pd.DataFrame(columns=['A','B','C','D','E','F','G'])
df.to_html('test.html')

产生:

<table border="1" class="dataframe">
  <thead>
    <tr style="text-align: right;">
      <th></th>
      <th>A</th>
      <th>B</th>
      <th>C</th>
      <th>D</th>
      <th>E</th>
      <th>F</th>
      <th>G</th>
    </tr>
  </thead>
  <tbody>
  </tbody>
</table>

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