Summarize some common mistakes of novices in Python development

The file name has the same name as the package name to be referenced

For example, you have to quote requests, But I'll name my file as requests.py, So execute the following code

import requests
requests.get('http://www.baidu.com')
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The following error will be reported

AttributeError: module 'requests' has no attribute 'get'
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The solution is for you python Change the name of the file , As long as it doesn't have the same package name , If you really don't want to change the file name , You can do it in the following way

import sys
_cpath_ = sys.path[0]
print(sys.path)
print(_cpath_)
sys.path.remove(_cpath_)
import requests
sys.path.insert(0, _cpath_)

requests.get('http://www.baidu.com')
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The main principle is to exclude the current directory from python Run the search directory , This processing is then executed on the command line python requests.py It can operate normally , But in pycharm The debugging and running of the system can't get through .

The problem of format misalignment

Here's the normal code

def fun():
    a=1
    b=2
    if a>b:
        print("a")  
    else:
        print("b")

fun()
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1. If else Not aligned

def fun():
    a=1
    b=2
    if a>b:
        print("a")  
     else:
        print("b")

fun()
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Will report

IndentationError: unindent does not match any outer indentation level
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2. If else and if There are no pairs , For example, write one directly else Or one more else, perhaps if and else The colon is missing

def fun():
    a=1
    b=2
    else:
        print("b")

fun()
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def fun():
    a=1
    b=2
    if a>b:
        print("a")
    else:
        print("b")
    else:
        print("b")

fun()
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def fun():
    a=1
    b=2
    if a>b:
        print("a")
    else
        print("b")

fun()
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Metropolitan newspaper

SyntaxError: invalid syntax
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3. If if and else The following statement is not indented

def fun():
    a=1
    b=2
    if a>b:
    print("a")
    else:
    print("b")

fun()
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Will report

IndentationError: expected an indented block
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The string uses Chinese quotation marks

For example, use Chinese quotation marks below

print(“a”)
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Will report

SyntaxError: invalid character in identifier
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The correct way is to use single quotation marks or double quotation marks in English

print('b')
print("b")
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It's no use calling the function , Mistakenly think that the function does not execute

Consider the following code

class A(object):
    def run(self):
        print('run')

a = A()
a.run
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The procedure is normal , There is no error during operation , But you may wonder why there is no printout , The reason, of course, is because a.run The reference of the returned function , It's not implemented , To be called a.run() Will find a printout . If you modify the code , Change to

class A(object):
    def run(self):
        return 1

a = A()
print(a.run+1)
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You will see the error report

TypeError: unsupported operand type(s) for +: 'method' and 'int'
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Change to a.run()+1 Then it's normal .

String formatting

This is a normal procedure

a = 1
b = 2
print('a = %s'%a)
print('a,b = %s,%s'%(a,b))
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1. If you write less %s

a = 1
b = 2
print('a = '%a) #  error 
print('a,b = %s'%(a,b)) #  error 
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Will be submitted to the

TypeError: not all arguments converted during string formatting
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2. If you write more %s

a = 1
b = 2
print('a = %s,%s'%a)
print('a,b = %s,%s,%s'%(a,b))
``
 Will be submitted to the 
```bash
TypeError: not enough arguments for format string
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3. If the formatted string is incorrect , For example, written in capital letters s

a = 1
b = 2
print('a = %S'%a)
print('a,b = %S,%S'%(a,b))
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Will be submitted to the

ValueError: unsupported format character 'S' (0x53) at index 5
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4. If % There's nothing in the back

a = 1
b = 2
print('a = %'%a)
print('a,b = %,%'%(a,b))
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Will be submitted to the

ValueError: incomplete format
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Yes None Error report of operation

For example, this code

a = [3,2,1]
b = a.sort()
print(a)
print(b)
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a The sorted value is [1,2,3],sort The return value of the function is None, therefore b Real time value of None, Not at all [1,2,3].

If you operate at this time b Take for example b The first element of

print(b[0])
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May be an error

TypeError: 'NoneType' object is not subscriptable
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Like copying arrays

print(b*2)
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May be an error

TypeError: unsupported operand type(s) for *: 'NoneType' and 'int'
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Such as execution extend function

b.extend([4])
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May be an error

AttributeError: 'NoneType' object has no attribute 'extend'
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Divide an integer by

python2 The default integer division is to return the integer part

print(3/2)
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Return is 1 and python3 It returns a decimal number

print(3/2)
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Return is 1.5 So the following code

a = [1,2,3]
print(a[len(a)/2])
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stay python2 You can get a value of 2, And in the python3 In the execution, you will get an error report

TypeError: list indices must be integers or slices, not float
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There are two ways to modify 1. Convert the result to an integer

a = [1,2,3]
print(a[int(len(a)/2)])
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2. use // Number

a = [1,2,3]
print(a[len(a)//2])
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These two expressions also apply to python2, Therefore, if you expect an integer to return, it is recommended to use the double division sign , Express your intention explicitly

The primary key does not exist

The dictionary obtained does not exist

a = {'A':1}
print(a['a'])
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Will report a mistake

KeyError: 'a'
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pip Network timeout while installing package

Similar to the following error bash WARNING: Retrying (Retry(total=3, connect=None, read=None, redirect=None, status=None)) after connection broken by 'ConnectTimeoutError(<pip._vendor.urllib3.connection.VerifiedHTTPSConnection object at 0x7f3b6ea319b0>, 'Connection to files.pythonhosted.org timed out. (connect timeout=15)')': /packages/xxxxxx It is recommended to set the image source , For example, replace it with Tsinghua source , Reference resources Portal

It's also possible to meet someone who doesn't add trusted-host The following error occurred due to parameter

Collecting xxx
The repository located at pypi.tuna.tsinghua.edu.cn is not a trusted or secure host and is being ignored. If this repository is available via HTTPS it is recommended to use HTTPS instead, otherwise you may silence this warning and allow it anyways with ‘--trusted-host pypi.tuna.tsinghua.edu.cn’.
Could not find a version that satisfies the requirement xxx (from versions: )
No matching distribution found for xxx
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Installation command added --trust-host that will do pip install -i pypi.tuna.tsinghua.edu.cn/simple --trusted-host pypi.tuna.tsinghua.edu.cn xxx