python变量NameError

Question

我很难将值赋值给vars，然后访问这些值。例如：

# offer users choice for how large of a song list they want to create
# in order to determine (roughly) how many songs to copy
print "\nHow much space should the random song list occupy?\n"
print "1. 100Mb"
print "2. 250Mb\n"

tSizeAns = raw_input()

if tSizeAns == 1:
    tSize = "100Mb"
elif tSizeAns == 2:
    tSize = "250Mb"
else:
    tSize = 100Mb    # in case user fails to enter either a 1 or 2

print "\nYou  want to create a random song list that is " + tSize + "."

回溯返回：

Traceback (most recent call last):
  File "./ranSongList.py", line 87, in <module>
    print "\nYou  want to create a random song list that is " + tSize + "."
NameError: name 'tSize' is not defined

我已经阅读了python变量，它们不需要声明，所以我认为它们可以被创建和动态使用，不是吗？如果是这样的话，我不太确定回溯是想告诉我什么。

顺便说一句，python似乎没有提供“case”功能，所以如果有人有任何建议，如何更好地向用户提供选择选项和分配var值的列表，我将非常感激地阅读它们。最后，当时间允许时，我将学习Tkinter并移植到GUI。

Answer 1

您的if语句正在检查int值。raw_input返回一个字符串。更改以下行：

tSizeAns = raw_input()

至

tSizeAns = int(raw_input())

Answer 2

除了上次100Mb中缺少的引号外，还需要引用if语句if tSizeAns == "1":中的常量，因为raw_input返回字符串，与整数相比，字符串总是返回false。

但是，缺少的引号并不是特定错误消息的原因，因为它会在执行之前导致语法错误。请检查您的张贴代码。我无法复制错误消息。

而且，在您使用if ... elif ... else的方式上，它基本上等同于其他语言中的case或switch，它既不降低可读性，也不长得多。这里可以用。如果您只想根据另一个值分配一个值，另一种可能是使用的方法是字典查找：

tSize = {"1": "100Mb", "2": "200Mb"}[tSizeAns]

但是，只有保证tSizeAns在tSize的范围内才能起作用。否则，您必须捕获KeyError异常或使用defaultdict：

lookup = {"1": "100Mb", "2": "200Mb"}
try:
    tSize = lookup[tSizeAns]
except KeyError:
    tSize = "100Mb"

或

from collections import defaultdict

[...]

lookup = defaultdict(lambda: "100Mb", {"1": "100Mb", "2": "200Mb"})
tSize = lookup[tSizeAns]

在您的例子中，我认为这些方法对于两个值是不合理的。但是，您可以使用字典同时构造初始输出。

Answer 3

这应该可以做到：

#!/usr/local/cpython-2.7/bin/python

# offer users choice for how large of a song list they want to create
# in order to determine (roughly) how many songs to copy
print "\nHow much space should the random song list occupy?\n"
print "1. 100Mb"
print "2. 250Mb\n"

tSizeAns = int(raw_input())

if tSizeAns == 1:
    tSize = "100Mb"
elif tSizeAns == 2:
    tSize = "250Mb"
else:
    tSize = "100Mb"    # in case user fails to enter either a 1 or 2

print "\nYou  want to create a random song list that is {}.".format(tSize)

顺便说一句，如果您愿意使用Python3.x，差别很小：

#!/usr/local/cpython-3.3/bin/python

# offer users choice for how large of a song list they want to create
# in order to determine (roughly) how many songs to copy
print("\nHow much space should the random song list occupy?\n")
print("1. 100Mb")
print("2. 250Mb\n")

tSizeAns = int(input())

if tSizeAns == 1:
    tSize = "100Mb"
elif tSizeAns == 2:
    tSize = "250Mb"
else:
    tSize = "100Mb"    # in case user fails to enter either a 1 or 2

print("\nYou want to create a random song list that is {}.".format(tSize))

HTH