source ： Open source in China
link ：https://www.oschina.net/question/89964_62779
For the newcomer Pythonista In the process of learning to run the code is more or less will encounter some errors , It may seem like a lot of work at first . As the amount of code accumulates , Practice makes perfect when encountering some runtime errors, it can quickly locate the original problem . Here are some common 17 A mistake , When you write code that doesn't make many of these mistakes , Yours Python The skill is on the next level . In other words , When you become a qualified Python After developer , You may be “ I can't write it ” This kind of mistake .
1
Forget in if,for,def,elif,else,class Wait until the end of the statement :
It can lead to “SyntaxError ：invalid syntax” as follows ：
if spam == 42
print('Hello!')
2
Use = instead of ==
It can also lead to “SyntaxError: invalid syntax”
= It's an assignment operator and == It's a comparison operation . This error occurs in the following code ：
if spam = 42:
print('Hello!')
3
Wrong use of indent
Lead to “IndentationError：unexpected indent”、“IndentationError：unindent does not match any outer indetation level” as well as “IndentationError：expected an indented block”
Remember that indent increase is only used to ： After the closing statement , And then you have to go back to the previous indent format . This error occurs in the following code ：
print('Hello!')
print('Howdy!')
perhaps ：
if spam == 42:
print('Hello!')
print('Howdy!')
4
stay for Forget to call... In the loop statement len()
Lead to “TypeError: 'list' object cannot be interpreted as an integer”
Usually you want to iterate through an index list perhaps string The elements of , This requires calling range() function . Remember to go back to len Value instead of returning this list .
This error occurs in the following code ：
spam = ['cat', 'dog', 'mouse']
for i in range(spam):
print(spam[i])
5
Try to modify string Value
Lead to “TypeError: 'str' object does not support item assignment”
string It's an immutable data type , This error occurs in the following code ：
spam = 'I have a pet cat.'
spam[13] = 'r'
print(spam)
And the right thing to do is ：
spam = 'I have a pet cat.'
spam = spam[:13] + 'r' + spam[14:]
print(spam)
6
Try to connect a non string value with a string
Lead to “TypeError: Can't convert 'int' object to str implicitly”
This error occurs in the following code ：
numEggs = 12
print('I have ' + numEggs + ' eggs.')
And the right thing to do is ：
numEggs = 12
print('I have ' + str(numEggs) + ' eggs.')
numEggs = 12
print('I have %s eggs.' % (numEggs))
7
Forget to quote at the beginning and end of the string
Lead to “SyntaxError: EOL while scanning string literal”
This error occurs in the following code ：
print(Hello!')
print('Hello!)
myName = 'Al'
print('My name is ' + myName + . How are you?')
8
Misspelling variable or function name
Lead to “NameError: name 'fooba' is not defined”
This error occurs in the following code ：
foobar = 'Al'
print('My name is ' + fooba)
spam = ruond(4.2)
spam = Round(4.2)
9
Misspelling method name
Lead to “AttributeError: 'str' object has no attribute 'lowerr'”
This error occurs in the following code ：
spam = 'THIS IS IN LOWERCASE.'
spam = spam.lowerr()
10
Reference exceeds list Maximum index
Lead to “IndexError: list index out of range”
This error occurs in the following code ：
spam = ['cat', 'dog', 'mouse']
print(spam[6])
11
Use a dictionary key that does not exist
Lead to “KeyError：‘spam’”
This error occurs in the following code ：
spam = { 'cat': 'Zophie', 'dog': 'Basil', 'mouse': 'Whiskers'}
print('The name of my pet zebra is ' + spam['zebra'])
12
Try to use Python Keyword as variable name
Lead to “SyntaxError：invalid syntax”
Python Key cannot be used as variable name , This error occurs in the following code ：
class = 'algebra'
Python3 The key words are ：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
Use the value-added operator... In a new variable definition
Lead to “NameError: name 'foobar' is not defined”
Do not use... When declaring variables 0 Or an empty string as the initial value , In this way, we use a sentence of the autoincrement operator spam += 1 be equal to spam = spam + 1, It means spam You need to specify a valid initial value .
This error occurs in the following code ：
spam = 0
spam += 42
eggs += 42
14
Use local variables in functions before defining local variables （ At this time, there is a global variable with the same name as the local variable ）
Lead to “UnboundLocalError: local variable 'foobar' referenced before assignment”
It's very complicated to use local variables in a function and have a global variable with the same name at the same time , The rule of use is ： If anything is defined in a function , If it's only used in functions, then it's local , On the contrary, it's a global variable .
This means that you can't use it as a global variable in a function before defining it .
This error occurs in the following code ：
someVar = 42
def myFunction():
print(someVar)
someVar = 100
myFunction()
15
Try to use range() Create a list of integers
Lead to “TypeError: 'range' object does not support item assignment”
Sometimes you want to get an ordered list of integers , therefore range() Looks like a good way to generate this list . However , You need to remember range() The return is “range object”, Not the actual list value .
This error occurs in the following code ：
spam = range(10)
spam[4] = -1
Write it correctly ：
spam = list(range(10))
spam[4] = -1
（ Be careful ： stay Python 2 in spam = range(10) It can be done , Because in Python 2 in range() The return is list value , But in Python 3 The above mistakes will occur in ）
16
non-existent ++ perhaps -- Self - increment and self - decrement operators .
Lead to “SyntaxError: invalid syntax”
If you are used to, for example C++ , Java , PHP Wait for other languages , Maybe you want to try to use ++ perhaps -- To increase or decrease by one variable . stay Python There is no such operator in .
This error occurs in the following code ：
spam = 1
spam++
Write it correctly ：
spam = 1
spam += 1
17
Forget to add... For the first parameter of the method self Parameters
Lead to “TypeError: myMethod() takes no arguments (1 given)”
This error occurs in the following code ：
class Foo():
def myMethod():
print('Hello!')
a = Foo()
a.myMethod()
After reading it , Believe that if you are rolling every day Python Words , Few of the above mistakes have been made .
Finally, what mistakes do you often make ？
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