python - 未绑定(bind)本地错误: local variable 'html' referenced before assignment

Question

html += '''
<table style="width:100%">
  <tr align="center">
    <th style="width:10%">Metrics</th>
    '''
def get_bus_metrics (met,name):
    for i in met:
        html += '<th>' + str(i) + '</th>'
    html += '''</tr>'''
    html += '''<tr><th>''' + name +'''</th>'''

get_bus_metrics (g1,'R')

UnboundLocalError: local variable 'html' referenced before assignment

我收到此错误。 有人可以建议我这里缺少什么，为什么我会收到上述错误。

Answer 1

如果之前未使用该变量，则修复 += 并将其提供给函数:

# fix here - no += unless you decleared html as some string beforehand
html = '''
<table style="width:100%">
  <tr align="center">
    <th style="width:10%">Metrics</th>
    '''
# fix here - else not known
def get_bus_metrics (met,name,html):
    for i in met:
        html += '<th>' + str(i) + '</th>'
    html += '''</tr>'''
    html += '''<tr><th>''' + name +'''</th>'''
    return html

html = get_bus_metrics (range(1,5),'R',html)  # provide as parameter: cleaner

print(html) # 

输出:

<table style="width:100%">
  <tr align="center">
    <th style="width:10%">Metrics</th>
    <th>1</th><th>2</th><th>3</th><th>4</th></tr><tr><th>R</th>

或者(不太可取)将其声明为全局:

def get_bus_metrics (met,name,html):
    # don't use globals, they are _evil_ - if you need to _modify_ smth
    # from global scope, you need to announce this in the function
    # reading works without if decleard earlier then the function, 
    # changing it needs this line:
    global html   
    for i in met:
        html += '<th>' + str(i) + '</th>'
    html += '''</tr>'''
    html += '''<tr><th>''' + name +'''</th>'''

---

提示 1:
使用 str.format() 更好的字符串格式或f-strings / PEP-0498 / Literal String Interpolation

提示 2:
在循环中添加字符串是一种浪费——它会构造大量被丢弃的中间字符串。使用列表代替

def get_bus_metrics (met,name,html):
    t = []
    for i in met:
        t.append('<th>{}</th>'.format(i))  # using format(..)
    t.append(f'</tr><tr><th>{name}</th>')  # using string interpol
    return html+''.join(t)                 # faster, less wasted intermediate strings

多库:

• str.format() - 适用于 2.7
• PEP-0498 string interpol适用于 3.6+
• Short Description of the Scoping Rules?如果您想了解范围界定