According toTheming and templates in Python, a figure can be created using a predefinedtemplate or updated to match it. However, the way updates happen is confusing if not inconsistent. Consider the examples below forplotly[express] == 6.2.0.

Plotly Express

importplotly.expressaspxdf=px.data.iris()# uses the default `plotly` templatefig=px.scatter(df,x="sepal_width",y="sepal_length",color="petal_length",title="Figure 1")fig.show()# update to use another template, note that the colour scale is unchangedfig.update_layout(template="seaborn",title="Figure 2")fig.show()# this is not equivalent to Figure 2fig=px.scatter(df,x="sepal_width",y="sepal_length",color="petal_length",template="seaborn",title="Figure 3")fig.show()

So Figure 3 is not the same as Figure 2, at least because the colour scales don't match. For custom themes, I can imagine that there may be other components of the figure object that would differ. As far as I know, there is no way to update traces using a template, e.g.,fig.update_traces(template="seaborn"), to adjust non-layout settings.

Plotly Graph Object

To make things more confusing, there is aSpecifying themes in graph object figures example in the documentation that does the same, but it actually works as expected. Adjusting the example we get:

importplotly.expressaspximportplotly.graph_objectsasgodf=px.data.iris()# uses the default `plotly` templatefig=go.Figure(data=go.Scatter(x=df["sepal_width"],y=df["sepal_length"],mode="markers",marker={"color":df["petal_length"],"showscale":True,        },    ))fig.update_layout(title="Figure 4")fig.show()# update to use another template, note that the colour scale has changed as expectedfig.update_layout(template="seaborn",title="Figure 5")fig.show()

However, specifying an explicitcolorscale will prevent a template update from setting its own colour scheme:

# same as Figure 4 but with an explicit `colorscale`fig=go.Figure(data=go.Scatter(x=df["sepal_width"],y=df["sepal_length"],mode="markers",marker={"color":df["petal_length"],"colorscale":"Viridis",# or any other colour scale"showscale":True,        },    ))fig.update_layout(title="Figure 6")fig.show()# no longer matches Figure 5fig.update_layout(template="seaborn",title="Figure 7")fig.show()

Suggestions

1. I would expectfig.update_layout(template="seaborn") to have the same effect, regardless of whetherpx orgo interface is used. From this perspective, to result for Figure 2 is a bug (the colour scale should have changed forseaborn template).
2. When an existing figure is updated, the settings from a template should take precedence over any existing settings. So Figure 5 and Figure 7 should produce the same result. This would entail that an update in Figure 7 overwrites whatever thecolorscale setting Figure 6 has with that of a template provided inupdate_layout.