python變量NameError

Question

我在為vars分配值然后訪問這些值時遇到麻煩。 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 + "."

追溯返回：

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

我已經閱讀了python變量，不需要聲明它們，因此我認為可以即時創建和使用它們，不是嗎？ 如果是這樣，我不太確定該回溯試圖告訴我什么。

順便說一句，似乎python不提供“大小寫”功能，因此，如果有人對如何更好地向用戶提供選擇選項和分配var值的建議，我將不勝感激。 最終，如果時間允許，我將學習Tkinter並移植到GUI。

Answer 1

除了周圍缺少引號100Mb的最后else ，你也想引用常量在if語句if tSizeAns == "1":因為raw_input返回一個字符串，它與整數比較將總是返回false 。

但是，缺少引號不是造成特定錯誤消息的原因，因為這將導致在執行之前出現語法錯誤。 請檢查您發布的代碼。 我無法重現該錯誤信息。

同樣， if ... elif ... else的使用方式，基本上等同於其他case或switch ，並且可讀性也不會更長。 可以在這里使用。 另一種方式可能是一個好主意，用，如果你只是想要分配基於另一個值的值是一個字典查找：

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

然而，這僅僅只要工作tSizeAns是保證在范圍tSize 。 否則，您將不得不捕獲KeyError異常或使用defaultdict：

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

要么

from collections import defaultdict

[...]

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

在您的情況下，我認為這些方法不適用於兩個值。 但是，您可以使用字典同時構造初始輸出。

Answer 2

您的if語句正在檢查int值。 raw_input返回一個字符串。 更改以下行：

tSizeAns = raw_input()

至

tSizeAns = int(raw_input())

Answer 3

應該這樣做：

#!/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)

順便說一句，如果您願意使用Python 3.x，則區別很小：

#!/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

Answer 4

初始化tSize為

tSize = "" 

在您的if區塊安全之前。 同樣在您的else情況下，將tSize放在引號中，以便它是字符串而不是int。 另外，您還在將字符串與整數進行比較。

Answer 5

我會這樣處理：

sizes = [100, 250]
print "How much space should the random song list occupy?"
print '\n'.join("{0}. {1}Mb".format(n, s)
                for n, s in enumerate(sizes, 1)) # present choices
choice = int(raw_input("Enter choice:")) # throws error if not int
size = sizes[0] # safe starting choice
if choice in range(2, len(sizes) + 1):
    size = sizes[choice - 1] # note index offset from choice
print "You  want to create a random song list that is {0}Mb.".format(size)

您也可以循環播放，直到獲得可接受的答案，並在出現錯誤的情況下掩蓋自己：

choice = 0
while choice not in range(1, len(sizes) + 1): # loop
    try: # guard against error
        choice = int(raw_input(...))
    except ValueError: # couldn't make an int
        print "Please enter a number"
        choice = 0
size = sizes[choice - 1] # now definitely valid

Answer 6

您忘記了一些報價：

# 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 + "."