Question

I am trying to format a table, such that data in each column are formatted in a style depending on their values (similar to conditional formatting in spreadsheet programs). How can I achieve that in pandas using the HTML formatter?

A typical use case is highlighting significant values in a table. For example:

    correlation  p-value
0   0.5          0.1
1   0.1          0.8
2   0.9          *0.01*

pandas allows to define custom formatters for HTML output – to obtain above output one could use:

import pandas as pd
from pandas.core import format
from StringIO import StringIO
buf = StringIO()
df = pd.DataFrame({'correlation':[0.5, 0.1,0.9], 'p_value':[0.1,0.8,0.01]})
fmt = format.DataFrameFormatter(df, 
          formatters={'p_value':lambda x: "*%f*" % x if x<0.05 else str(x)})
format.HTMLFormatter(fmt).write_result(buf)

However, I would like to change the style for significant values (for example, by using bold font).

A possible solution would be to attach a CSS class to <td> tags in the HTML output, which could be then formatted using CSS stylesheet. The above would then become:

<table border="1" class="dataframe">
  <thead>
    <tr style="text-align: right;">
      <th></th>
      <th>correlation</th>
      <th>p_value</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>0</td>
      <td> 0.5</td>
      <td> 0.10</td>
    </tr>
    <tr>
      <td>1</td>
      <td> 0.1</td>
      <td> 0.80</td>
    </tr>
    <tr>
      <td>2</td>
      <td> 0.9</td>
      <td class='significant'> 0.01</td>
    </tr>
  </tbody>
</table>

Edit: As suggested by @Andy-Hayden I can add formatting by simply replacing stars with <span class="signifcant">...</span> in my example:

import pandas as pd
from StringIO import StringIO
buf = StringIO()
significant = lambda x: '<span class="significant">%f</span>' % x if x<0.05 else str(x)
df = pd.DataFrame({'correlation':[0.5, 0.1,0.9], 'p_value':[0.1,0.8,0.01]})
df.to_html(buf, formatters={'p_value': significant})

Newer versions of pandas escape the tags. To avoid it replace last line with:

df.to_html(buf, formatters={'p_value': significant}, escape=False)

Answer 1

You can use the DataFrame to_html method, which comes with formatters argument.

An easier solution would be to surround by <span class="significant"> and </span>, (rather than *). Note: by default this will be escaped (i.e. < becomes <) so you will need to use the escape=False argument.