dcc.Dropdown value returns unicode in Python 2 and string in Python 3

[shammamah-zz]

Seen in https://github.com/plotly/dash-docs/pull/813

Minimal example:

# -*- coding: utf-8 -*-
import dash
from dash.dependencies import Input, Output
import dash_core_components as dcc
import dash_html_components as html
import pandas as pd

import plotly.express as px

app = dash.Dash(__name__)

df = pd.read_csv('https://raw.githubusercontent.com/plotly/datasets/master/gapminderDataFiveYear.csv')

available_countries = df['country'].unique()

app.layout = html.Div([
    dcc.Graph(
        id='graph-px'
    ),
    'Indicator',
    dcc.Dropdown(
        id='graph-indicator-px',
        options=[
            {'label': 'Population', 'value': 'pop'},
            {'label': 'Life Expectancy', 'value': 'lifeExp'},
            {'label': 'GDP per Capita', 'value': 'gdpPercap'}
        ],
        value='pop'
    ),
    html.Div(id='indicator-type')
])


@app.callback(
    Output('graph-px', 'figure'),
    [Input('graph-indicator-px', 'value')]
)
def update_store_data(indicator):
    dff = df[df['country'] == 'Canada']
    return px.scatter(dff, x='year', y=indicator)


@app.callback(
    Output('indicator-type', 'children'),
    [Input('graph-indicator-px', 'value')]
)
def update_indicator_type(indicator):
    return str(type(indicator))


if __name__ == '__main__':
    app.run_server(debug=True)

python-2.7:

python-3.7:

Consequently, the column name (set by the dropdown) gets interpreted as an iterable itself, and if the length of the column name is not the same as the number of elements in the column, there is an error in plotly.express below:
https://github.com/plotly/plotly.py/blob/60e366645138836cbaf3122e3033d8db35d9caf9/packages/python/plotly/plotly/express/_core.py#L1016

[alexcjohnson]

This issue isn't specific to dropdowns, it's just the behavior of json.loads in Py2, any string will come back as unicode:

>>> json.loads('{"say":"hello"}')
{u'say': u'hello'}

I'm tempted to just ignore this and say "if you're still on Py2 it's your problem disentangling strings & unicode," but ping @nicolaskruchten to see if it's worth accepting unicode column names in px while the underlying dataframe has string names (or vice versa, if that's a thing)?

I'll note for reference that dict keys in Py2 gloss over the string/unicode distinction:

>>> a=json.loads('{"say":"hello"}')
>>> a['say']
u'hello'
>>> a[u'say']
u'hello'

>>> b={'y': 1, u'y': 2}
>>> b
{'y': 2}