我无法循环访问下拉小部件输出

Question

我已经兜圈子好几个小时了。我正在尝试让下拉输出循环，以确保结果正确。

我得到了下拉列表，但是 "output" 是 none。如果我 select 'DEV' 或 "DEV"，它会打印 DEV。输出 (w) 是 none 并且循环退出 else not if??

python代码（jypter）：

source = ["Select Source", "DEV", "TEMP", "PROD"]
source_ = widgets.Dropdown(
    options=source,
    value=source[0],
    description='Select variable:',
    disabled=False,
    button_style=''
)
def sourceURL(b):
    clear_output()
    print(source_.value)

### Drop Down
print("Drop Down")
display(source_)
w = source_.observe(sourceURL)

## print("output: ")
print(w)            ### output is None

#### LOOP
if w == 'DEV':
    print("This is Dev")    
elif w == "TEST":
    print("This is TEST")  
else:
    print("This is PROD")

Answer 1

当您执行 source_.observe(sourceURL) 时，此调用没有 return 值。因此这相当于 w = None.

要获得您想要的行为，我认为您需要将脚本末尾的代码移动到 sourceURL 函数中。

import ipywidgets as widgets
from IPython.display import clear_output

source = ["DEV", "TEMP", "PROD"]
source_ = widgets.Dropdown(
    options=source,
    value=source[0],
    description='Select variable:',
    disabled=False,
    button_style=''
)
def sourceURL(b):
#     clear_output()
    w = b['new']

    if w == 'DEV':
        print("This is Dev")    
    elif w == "TEMP":
        print("This is TEMP")  
    else:
        print("This is PROD")

### Drop Down
print("Drop Down")
display(source_)
w = source_.observe(sourceURL, names='value')

我无法循环访问下拉小部件输出

I cannot access dropdown widget output in a loop

widget

python-3.6

ipywidgets