How to combine .add_annotation and go.Layout in plotly?

Question

How to combine the command displaying text with go.Layout, please? The command fig.add_annotation stop working when go.Layout() is present. go.Layout includes many features; thus, I would like to not change it.

import pandas as pd
import plotly.express as px
import plotly.graph_objects as go

## sample DataFrames
df1=pd.DataFrame({'A':[1,2,3],'B':[4,5,6]})


fig = px.scatter(df1, x='A', y='B')

fig.add_annotation(text="Absolutely-positioned annotation",
                  x=2.5, y=4.5, showarrow=False)

layout = go.Layout(
    template = "plotly_white",
    title="<b>O-C diagram</b>",
    font_family="Trebuchet",
    title_font_family="Trebuchet",
    title_font_color="Navy",
    xaxis_tickformat = "5.0",
    yaxis_tickformat = ".1f",
    xaxis2 = XAxis( 
        overlaying='x',
        side='top',
    ),
    legend=dict(
        yanchor="top",
        y=0.99,
        xanchor="left",
        x=0.83,
    font=dict(
        family="Trebuchet",
        size=20,
        color="black"
     ),
        bgcolor="LightGray",
        bordercolor="Black",
        borderwidth=0
     ),
    xaxis_title=r'<i>T</i>',
    yaxis_title=r'O',
    title_x=0.5,
    font=dict(
        family="Trebuchet",
        size=26,
        color="Black"
    ),
)

fig.layout = layout

fig.show()

Answer 1

The problem here isn't that you're using:

layout = go.Layout(
    template = "plotly_white",
)

But rather that...

fig.layout = layout

...overrides every layout attribute other than template.

If you instead use:

fig.update_layout(template = "plotly_white")

Then you'll get:

Complete code:

import pandas as pd
import plotly.express as px
import plotly.graph_objects as go

## sample DataFrames
df1=pd.DataFrame({'A':[1,2,3],'B':[4,5,6]})


fig = px.scatter(df1, x='A', y='B')


fig.add_annotation(text="Absolutely-positioned annotation",
                  x=2.5, y=4.5, showarrow=False)

# layout = go.Layout(
#     template = "plotly_white",
# )

# fig.layout = layout
fig.update_layout(template = "plotly_white")
fig.show()

Edit - Another suggestion

If you for some reason need to stick with your original setup, then just change the order you're assigning the template and the annotations to your figure:

import pandas as pd
import plotly.express as px
import plotly.graph_objects as go

## sample DataFrames
df1=pd.DataFrame({'A':[1,2,3],'B':[4,5,6]})


fig = px.scatter(df1, x='A', y='B')


layout = go.Layout(
    template = "plotly_white",
)

fig.layout = layout
fig.add_annotation(text="Absolutely-positioned annotation",
                  x=2.5, y=4.5, showarrow=False)

fig.show()