python study notes 5

Chat 9 ImportAndException

Python can write to py files and run files directly

%%writefile ex1.py
PI = 3.1416

def sum(lst):
    tot = lst[0]
    for value in lst[1:]:
        tot = tot + value
    return tot
    
w = [0, 1, 2, 3]
print sum(w), PI

%run ex1.py

Of course I can also import this file directly

import ex1
print ex1.PI
reload(ex1)
# 重新载入ex1,这样可以重新

import os
os.remove('ex1.py')
# 删除这个文件

Sometimes we want to use a .pyfile as both a script and a module, and __name__this property can be used at this time.

__name__The value will only be if the file is being executed as a script '__main__', so we can:

if __name__ == '__main__':

Suppose we have a folder like this:

foo/

• __init__.py
• bar.py(defines func)
• baz.py(defines zap)

Import package requirements:

• folder fooin Python 's search path
• __init__.pyIndicates that foois a package, which can be an empty file.

It seems that there is no need to write something in python3

python2 needs to write import bar, baz

---

Introduce the exception

If the following function is negative, there will be a bug

import math

while True:
    text = raw_input('> ')
    if text[0] == 'q':
        break
    x = float(text)
    y = math.log10(x)
    print "log10({0}) = {1}".format(x, y)

we can use try

import math

while True:
    try:
        text = raw_input('> ')
        if text[0] == 'q':
            break
        x = float(text)
        y = math.log10(x)
        print "log10({0}) = {1}".format(x, y)
    except ValueError:
        print "the value must be greater than 0"

import math

while True:
    try:
        text = raw_input('> ')
        if text[0] == 'q':
            break
        x = float(text)
        y = 1 / math.log10(x)
        print "1 / log10({0}) = {1}".format(x, y)
    except Exception:
        print "invalid value"

import math

while True:
    try:
        text = raw_input('> ')
        if text[0] == 'q':
            break
        x = float(text)
        y = 1 / math.log10(x)
        print "1 / log10({0}) = {1}".format(x, y)
    except (ValueError, ZeroDivisionError):
        print "invalid value"

import math

while True:
    try:
        text = raw_input('> ')
        if text[0] == 'q':
            break
        x = float(text)
        y = 1 / math.log10(x)
        print "1 / log10({0}) = {1}".format(x, y)
    except ValueError:
        print "the value must be greater than 0"
    except ZeroDivisionError:
        print "the value must not be 1"
# 这样就可以像if elif一样

import math

while True:
    try:
        text = raw_input('> ')
        if text[0] == 'q':
            break
        x = float(text)
        y = 1 / math.log10(x)
        print "1 / log10({0}) = {1}".format(x, y)
    except ValueError as exc:
        if exc.message == "math domain error":
            print "the value must be greater than 0"
        else:
            print "could not convert '%s' to float" % text
    except ZeroDivisionError:
        print "the value must not be 1"
    except Exception as exc:
        print "unexpected error:", exc.message

When we except Exceptionuse , all Exceptionand its derived subclasses are captured, but not all exceptions are derived from the Exceptionclass , and there may be some situations that cannot be captured. Therefore, a more general approach is to use this form :

try:
    pass
except:
    pass

Such unspecified exception type will catch all exceptions, but this form is not recommended.

We can also use finally, that is, no matter what, it will be executed

try:
    print 1 / 0
except ZeroDivisionError:
    print 'divide by 0.'
finally:
    print 'finally was called.'

Chat10 Warning and IO

There are cases where I want the program to keep running, and then I need the user to know the following things

import warnings

def month_warning(m):
    if not 1<= m <= 12:
        msg = "month (%d) is not between 1 and 12" % m
        warnings.warn(msg, RuntimeWarning)

month_warning(13)

warnings.filterwarnings(action = 'ignore', category = RuntimeWarning)

month_warning(13)

---

The following describes the IO operation

Write the test file:

%%writefile test.txt
this is a test file.
hello world!
python is good!
today is a good day.

read file

f = open('test.txt')
f = file('test.txt')

text = f.read()
print text

f = open('test.txt')
lines = f.readlines()
print lines
# readlines 方法返回一个列表，每个元素代表文件中每一行的内容：
f.close()

write file

f = open('myfile.txt', 'w')
f.write('hello world!')
f.close()

print open('myfile.txt').read()

f = open('myfile.txt', 'w+')
f.write('hello world!')
f.seek(6)
print f.read()
f.close()
# seek就是移动到第4个字符