在为我的@app.route ("/buy") 运行 Python 代码时，我无法理解所遇到的 ValueError。有人可以解释什么是ValueError: View function did not return a response以及如何避免这种情况？此外，您如何能够通过给出的错误消息了解您的问题发生在哪里。非常感谢！
Traceback (most recent call last):
File "/usr/local/lib/python3.4/dist-packages/flask/app.py", line 1994, in __call__
return self.wsgi_app(environ, start_response)
File "/usr/local/lib/python3.4/dist-packages/flask/app.py", line 1985, in wsgi_app
response = self.handle_exception(e)
File "/usr/local/lib/python3.4/dist-packages/flask/app.py", line 1540, in handle_exception
reraise(exc_type, exc_value, tb)
File "/usr/local/lib/python3.4/dist-packages/flask/_compat.py", line 33, in reraise
raise value
File "/usr/local/lib/python3.4/dist-packages/flask/app.py", line 1982, in wsgi_app
response = self.full_dispatch_request()
File "/usr/local/lib/python3.4/dist-packages/flask/app.py", line 1615, in full_dispatch_request
return self.finalize_request(rv)
File "/usr/local/lib/python3.4/dist-packages/flask/app.py", line 1630, in finalize_request
response = self.make_response(rv)
File "/usr/local/lib/python3.4/dist-packages/flask/app.py", line 1725, in make_response
raise ValueError('View function did not return a response')
ValueError: View function did not return a response
（“/买”）：
@app.route("/buy", methods=["GET", "POST"])
@login_required
def buy():
"""Buy shares of stock."""
if request.method == "POST":
#get symbol from user
symbl = request.form.get("symbol")
if not symbl:
return apology ("Please insert a symbol")
while True:
#number of shares to buy
num = request.form.get("number")
try:
number = float (num)
except ValueError:
return apology ("Please insert valid number of stocks you'd like
enter code hereto buy")
continue
else:
break
if number is None or number == '' or number < 1:
return apology ("Please insert valid number of stocks you'd like
to buy")
#lookup and save dict in quoted
quoted = lookup(symbl)
#if symbl is invalid return apology
if not quoted:
return apology ("Not a valid stock")
else:
#quotedprice saves price of share
quotedprice=quoted["price"]
#price of a single share * the number of shares required to buy
prc=float(qtd)*number
#remember session id
ide = session["user_id"]
csh=db.execute("SELECT * FROM users WHERE id = :ide", ide=ide)
#continue if user has enough
if prc <= csh[0]["cash"]:
db.execute("INSERT INTO portfolio (id, symbol, price, shares,
action, dtime) VALUES (:ide, :symbol, :price, :shares, 'Buy',
DateTime('now'))", ide=ide, symbol = symbl, price = prc,
shares = number)
db.execute("UPDATE users SET cash = :cash WHERE id = :ide",
cash = csh[0]["cash"] - prc, ide = ide)
return redirect(url_for("index"))
else:
return apology ("Not enough cash to purchase stocks")
else:
return render_template("buy.html")
购买.html：
{% extends "layout.html" %}
{% block title %}
Please enter the Symbol to buy the stock.
{% endblock %}
{% block main %}
<form action="{{ url_for('buy') }}" method="post">
<fieldset>
<div class="form-group">
<input autocomplete="off" autofocus class="form-control" name="symbol" placeholder="Stock Symbol" type="text"/>
</div>
<div class="form-group">
<input autocomplete="off" autofocus class="form-control" name="number" placeholder="Amount of Stock" type="text"/>
</div>
<div class="form-group">
<button class="btn btn-default" type="submit">Submit</button>
</div>
</fieldset>
</form>
{% endblock %}