新手必看，$17$ 个常见的 Python 运行时错误

Python

英文原文：

https://inventwithpython.com/blog/2012/07/09/16-common-python-runtime-errors-beginners-find/

初入门的 Pythoner 在运行代码时免不了会遇到一些错误，刚开始可能看起来比较费劲。

随着代码量的积累，熟能生巧，当遇到一些运行时错误时能够很快的定位问题原题。

下面整理了常见的 $17$ 个错误，希望能够帮助到大家。

1、

忘记在 if，for，def，elif，else，class 等声明末尾加 :会导致 SyntaxError ：

1) Forgetting to put a ":" at the end of an if, elif, else, for, while, class, or def statement. (Causes “SyntaxError: invalid syntax”)

invalid syntax 如下：
This error happens with code like this:

if spam == 42
  print('Hello!')

2、

使用 = 而不是 == 也会导致 SyntaxError: invalid syntax
Using = instead of ==. (Causes “SyntaxError: invalid syntax”)

The = is the assignment operator while == is the "is equal to" comparison operator.
= 是赋值操作符，而 == 是等于比较操作

This error happens with code like this:
该错误发生在如下代码中：

if spam = 42:
   print('Hello!')

3、

错误的使用缩进量导致
3) Using the wrong amount of indentation. (Causes “IndentationError: unexpected indent” and “IndentationError: unindent does not match any outer indentation level” and “IndentationError: expected an indented block”)

Remember that the indentation only increases after a statement ending with a : colon, and afterwards must return to the previous indentation.
IndentationError：unexpected indent

IndentationError：unindent does not match any outer indetation level

以及IndentationError：expected an indented block

记住缩进增加只用在以:结束的语句之后，而之后必须恢复到之前的缩进格式。

该错误发生在如下代码中：
This error happens with code like this:

print('Hello!')
   print('Howdy!')

或者：

if spam == 42:
    print('Hello!')
print('Howdy!')

4、

在 for 循环语句中忘记调用 len()导致 TypeError: 'list' object cannot be interpreted as an integer
4) Forgetting the len() call in a for loop statement. (Causes “TypeError: 'list' object cannot be interpreted as an integer”)

通常你想要通过索引来迭代一个 list 或者 string 的元素，这需要调用 range() 函数。要记得返回 len 值而不是返回这个列表。
Commonly you want to iterate over the indexes of items in a list or string, which requires calling the range() function. Just remember to pass the return value of len(someList), instead of passing just someList.

该错误发生在如下代码中：
This error happens with code like this:

spam = ['cat', 'dog', 'mouse']
for i in range(spam):
    print(spam[i])

5、

尝试修改 string 的值，导致 TypeError: 'str' object does not support item assignment
5) Trying to modify a string value. (Causes “TypeError: 'str' object does not support item assignment”)

string 是一种不可变的数据类型，该错误发生在如下代码中：
Strings are an immutable data type. This error happens with code like this:

spam = 'I have a pet cat.'
spam[13] = 'r'
print(spam)

而正确做法是：

spam = 'I have a pet cat.'
spam = spam[:13] + 'r' + spam[14:]
print(spam)

6、

尝试连接非字符串值与字符串（导致 TypeError: Can't convert 'int' object to str implicitly）
6) Trying to concatenate a non-string value to a string value. (Causes “TypeError: Cannot convert 'int' object to str implicitly”)

该错误发生在如下代码中：
This error happens with code like this:

numEggs = 12
print('I have ' + numEggs + ' eggs.')

而正确做法是：

numEggs = 12
print('I have ' + str(numEggs) + ' eggs.')

numEggs = 12
print('I have %s eggs.' % (numEggs))

7、

在字符串首尾忘记加引号（导致 SyntaxError: EOL while scanning string literal）
7) Forgetting a quote to begin or end a string value. (Causes “SyntaxError: EOL while scanning string literal”)

This error happens with code like this:
该错误发生在如下代码中：

print(Hello!')
print('Hello!)
myName = 'Al'
print('My name is ' + myName + . How are you?')

8、

变量或者函数名拼写错误（导致 NameError: name 'fooba' is not defined）
8) A type for a variable or function name. (Causes “NameError: name 'fooba' is not defined”)

This error happens with code like this:
该错误发生在如下代码中：

foobar = 'Al'
print('My name is ' + fooba)
spam = ruond(4.2)
spam = Round(4.2)

9、

方法名拼写错误（导致 AttributeError: 'str' object has no attribute 'lowerr'）
9) A typo for a method name. (Causes “AttributeError: 'str' object has no attribute 'lowerr'”)

该错误发生在如下代码中：
This error happens with code like this:

spam = 'THIS IS IN LOWERCASE.'
spam = spam.lowerr()

10、

引用超过 list 最大索引（导致 IndexError: list index out of range）
10) Going past the last index of a list. (Causes “IndexError: list index out of range”)

该错误发生在如下代码中：
This error happens with code like this:

spam = ['cat', 'dog', 'mouse']
print(spam[6])

11、

使用不存在的字典键值（导致 KeyError：‘spam’）
11) Using a non-existent dictionary key. (Causes “KeyError: 'spam'”)

该错误发生在如下代码中：
This error happens with code like this:

spam = {'cat': 'Zophie', 'dog': 'Basil', 'mouse': 'Whiskers'}
print('The name of my pet zebra is ' + spam['zebra'])

12、

尝试使用 Python 关键字作为变量名（导致 SyntaxError：invalid syntax）
12) Trying to use a Python keyword for a variable name. (Causes “SyntaxError: invalid syntax”)

Python 关键不能用作变量名，该错误发生在如下代码中：
The Python keywords (also called reserved words) cannot be used for variable names. This happens with code like:

class = 'algebra'

Python3 的关键字有：

and, as, assert, break, class, continue, def, del, elif, else, except, False, finally, for, from, global, if, import, in, is, lambda, None, nonlocal, not, or, pass, raise, return, True, try, while, with, yield

13、

在一个定义新变量中使用增值操作符

导致NameError: name 'foobar' is not defined

不要在声明变量时使用 0 或者空字符串作为初始值，这样使用自增操作符的一句spam += 1等于spam = spam + 1，这意味着 spam 需要指定一个有效的初始值。

该错误发生在如下代码中：

spam = 0
spam += 42
eggs += 42

14、

在定义局部变量前在函数中使用局部变量（此时有与局部变量同名的全局变量存在）
14) Using a local variable (with the same name as a global variable) in a function before assigning the local variable. (Causes “UnboundLocalError: local variable 'foobar' referenced before assignment”)

Using a local variable in a function that has the same name as a global variable is tricky. The rule is: if a variable in a function is ever assigned something, it is always a local variable when used inside that function. Otherwise, it is the global variable inside that function.

This means you cannot use it as a global variable in the function before assigning it.

导致UnboundLocalError: local variable 'foobar' referenced before assignment

在函数中使用局部变量而同时又存在同名全局变量时是很复杂的，使用规则是：

如果在函数中定义了任何东西，如果它只是在函数中使用那它就是局部的，反之就是全局变量。

这意味着你不能在定义它之前把它当全局变量在函数中使用。

该错误发生在如下代码中：
This error happens with code like this:

someVar = 42
def myFunction():
    print(someVar)
    someVar = 100
myFunction()

15、

尝试使用 range() 创建整数列表，导致TypeError: 'range' object does not support item assignment
15) Trying to use range() to create a list of integers. (Causes “TypeError: 'range' object does not support item assignment”)

有时你想要得到一个有序的整数列表，所以 range() 看上去是生成此列表的不错方式。然而，你需要记住 range() 返回的是 range object，而不是实际的 list 值。
Sometimes you want a list of integer values in order, so range() seems like a good way to generate this list. However, you must remember that range() returns a "range object", and not an actual list value.

该错误发生在如下代码中：
This error happens with code like this:

spam = range(10)
spam[4] = -1

正确写法：

spam = list(range(10))
spam[4] = -1

注意：在 Python 2 中 spam = range(10) 是能行的，因为在 Python 2 中 range() 返回的是 list 值，但是在 Python 3 中就会产生以上错误.

16、

不存在 ++ 或者 -- 自增自减操作符，导致SyntaxError: invalid syntax
16) There is no ++ increment or –- decrement operator. (Causes “SyntaxError: invalid syntax”)

如果你习惯于例如 C++ , Java , PHP 等其他的语言，也许你会想要尝试使用++ 或者 -- 自增自减一个变量。在 Python 中是没有这样的操作符的。
If you come from a different programming language like C++, Java, or PHP, you may try to increment or decrement a variable with ++ or --. There are no such operators in Python.

该错误发生在如下代码中：
This error happens with code like this:

spam = 1
spam++

正确写法：

spam = 1
spam += 1

17、

忘记为方法的第一个参数添加 self 参数（导致 TypeError: myMethod() takes no arguments (1 given)）
17) Update: As Luciano points out in the comments, it is also common to forget adding self as the first parameter for a method. (Causes “ TypeError: myMethod() takes no arguments (1 given) ”)

This error happens with code like this:
该错误发生在如下代码中：

class Foo():
    def myMethod():
        print('Hello!')
a = Foo()
a.myMethod()