cs50金融项目:lookup()不返回字典

Question

在实现buy()函数时，我需要使用从helpers.py中找到的lookup(symbol)函数返回的字典，并在app.py中访问它。但它似乎没有返回任何数据。代码如下:

# in helper.py

"""
Decorate routes to require login.
https://flask.palletsprojects.com/en/1.1.x/patterns/viewdecorators/
"""
@wraps(f)
def decorated_function(*args, **kwargs):
if session.get("user_id") is None:
return redirect("/login")
return f(*args, **kwargs)
return decorated_function

def lookup(symbol):
"""Look up quote for symbol."""
# Contact API
try:
api_key = os.environ.get("API_KEY")
response = requests.get(f"https://cloud-sse.iexapis.com/stable/stock/{urllib.parse.quote_plus(symbol)}/quote?token={api_key}")
response.raise_for_status()
except requests.RequestException:
return None
# Parse response
try:
quote = response.json()
return {
"name": quote["companyName"],
"price": float(quote["latestPrice"]),
"symbol": quote["symbol"]
}
except (KeyError, TypeError, ValueError):
return None

， app.py中的代码是

def buy():
if request.method == "GET":
return render_template("buy.html")
else:
symbol = request.form.get("symbol")
shares = request.form.get("share_num")
result = lookup(symbol)

cash = db.execute("SELECT cash FROM users WHERE id = ?", session["user_id"])
if cash < result["price"] * request.form.get("share_num"):
return apology("insufficient balance",403)
db.execute("INSERT INTO stock(symbol,name,quantity,price) VALUES (?,?,?,?) WHERE users.id = stock.user_id AND id= ?",
result["symbol"], result["name"], request.form.get("share_num"),result["price"],session["user_id"])
db.execute("UPDATE users(cash) SET (?) WHERE users.id = stock.user_id AND id=? ",
cash - result["price"] * request.form.get("share_num"), session["user_id"])
return redirect("/")

和HTML代码是

{% block title %}
buy stock
{% endblock %}
{% block main %}
<form action="/buy" method="post">
<div class="mb-3">
<input autocomplete="off" autofocus class="form-control mx-auto w-auto"  name="symbol" placeholder="symbol" type="text">
</div>
<div class="mb-3">
<input class="form-control mx-auto w-auto" name="share_num" placeholder="quantity" type="number">
</div>
<button class="btn btn-primary" type="submit">buy</button>
</form>
{% endblock %}

得到的错误是

File "/workspaces/89966944/finance/helpers.py", line 33, in decorated_function
return f(*args, **kwargs)
^^^^^^^^^^^^^^^^^^
File "/workspaces/89966944/finance/app.py", line 64, in buy
if cash < result["price"] * request.form.get("share_num"):
~~~~~~^^^^^^^^^
TypeError: 'NoneType' object is not subscriptable

Answer 1

错误提示您正在尝试获取None对象中的键。

在本例中，result为None，您正在尝试执行result["price"]，这会触发TypeError。

注意lookup函数可以返回None，如果:

• API有连接问题(第一次尝试除了负责检查这个)
• API返回的响应不是字典。例如，如果你输入一个不存在的符号，就会发生这种情况。(第二次尝试除外负责此)

在调用result["price"]之前，必须检查lookup是否返回None，以避免获得KeyErrors。例如:

result = lookup(symbol)
if result is None:
# Do something if lookup returns None

确保使用浏览器调用API来检查它返回的是哪种类型的数据。