How to change the datetime format in pandas

Question

My dataframe has a DOB column (example format 1/1/2016) which by default gets converted to pandas dtype \'object\': DOB object

Answer 1

There is a difference between

• the content of a dataframe cell (a binary value) and
• its presentation (displaying it) for us, humans.

So the question is: How to reach the appropriate presentation of my datas without changing the data / data types themselves?

Here is the answer:

• If you use the Jupyter notebook for displaying your dataframe, or
• if you want to reach a presentation in the form of an HTML file (even with many prepared superfluous id and class attributes for further CSS styling — you may or you may not use them),

use styling. Styling don't change data / data types of columns of your dataframe.

Now I show you how to reach it in the Jupyter notebook — for a presentation in the form of HTML file see the note near the end of the question.

I will suppose that your column DOB already has the type datetime64 (you shown that you know how to reach it). I prepared a simple dataframe (with only one column) to show you some basic styling:

• Not styled:

     df
  

          DOB
0  2019-07-03
1  2019-08-03
2  2019-09-03
3  2019-10-03

• Styling it as mm/dd/yyyy:

     df.style.format({"DOB": lambda t: t.strftime("%m/%d/%Y")})
  

          DOB
0  07/03/2019
1  08/03/2019
2  09/03/2019
3  10/03/2019

• Styling it as dd-mm-yyyy:

     df.style.format({"DOB": lambda t: t.strftime("%d-%m-%Y")}) 
  

          DOB
0  03-07-2019
1  03-08-2019
2  03-09-2019
3  03-10-2019

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Be careful!
The returning object is NOT a dataframe — it is an object of the class Styler, so don't assign it back to df:

Don´t do this:

df = df.style.format({"DOB": lambda t: t.strftime("%m/%d/%Y")})    # Don´t do this!

(Every dataframe has its Styler object accessible by its .style property, and we changed this df.style object, not the dataframe itself.)

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Questions and Answers:

• Q: Why your Styler object (or an expression returning it) used as the last command in a Jupyter notebook cell displays your (styled) table, and not the Styler object itself?

• A: Because every Styler object has a callback method ._repr_html_() which returns an HTML code for rendering your dataframe (as a nice HTML table).

  Jupyter Notebook IDE calls this method automatically to render objects which have it.

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Note:

You don't need the Jupyter notebook for styling (i.e. for nice outputting a dataframe without changing its data / data types).

A Styler object has a method render(), too, if you want to obtain a string with the HTML code (e.g. for publishing your formatted dataframe to the Web, or simply present your table in the HTML format):

df_styler = df.style.format({"DOB": lambda t: t.strftime("%m/%d/%Y")})
HTML_string = df_styler.render()