Python 中的系统 vs 调用 vs popen [英] system vs call vs popen in Python

问题描述

cmd = 'touch -d '+date_in+' '+images_dir+'/'+photo_name
os.system(cmd)

不起作用

subprocess.call(['touch','-d','{}'.format(date_in),'{}'.format(images_dir+'/'+photo_name)])

不起作用

subprocess.Popen(['touch','-d','{}'.format(date_in),'{}'.format(images_dir+'/'+photo_name)])

有效！

为什么?在前两种情况下我遗漏了什么?

Why? What am I missing in first two cases?

pi@raspberrypi:~ $ python --version
Python 2.7.13

实际代码片段:

try:
    response = urllib2.urlopen(url)
    if(response.getcode() == 200):
        photo_file = response.read()
        with open(images_dir+'/'+photo_name,'wb') as output:
            output.write(photo_file)
            #cmd = 'touch -d '+date_in+' '+images_dir+'/'+photo_name
            #subprocess.Popen(['touch','-d','{}'.format(date_in),'{}'.format(images_dir+'/'+photo_name)])
            subprocess.check_call(['touch','-d','{}'.format(date_in),'{}'.format(images_dir+'/'+photo_name)])
        with open(images_dir+'/captions/'+photo_name+'.txt','wb') as output:
            output.write(photo_title)
    else:
        print 'Download error'
except Exception as message:
    print 'URL open exception {}'.format(message)

推荐答案

现在很清楚了:

with open(images_dir+'/'+photo_name,'wb') as output:
    output.write(photo_file)
    #cmd = 'touch -d '+date_in+' '+images_dir+'/'+photo_name
    #subprocess.Popen(['touch','-d','{}'.format(date_in),'{}'.format(images_dir+'/'+photo_name)])
    subprocess.check_call(['touch','-d','{}'.format(date_in),'{}'.format(images_dir+'/'+photo_name)])

我的假设是:你仍然在 with 块中，所以如果 check_call 或 system 被执行，它结束，然后文件被关闭，再次设置日期并破坏 touch 工作.

My assumption is: you're still in the with block, so if check_call or system is performed, it ends, then the file is closed, setting the date again and ruining touch efforts.

使用Popen，进程在后台执行，所以当它执行时，文件已经关闭(嗯，实际上这是一个竞争条件，不能保证)

With Popen, the process is performed in the background, so when it's executed, the file is already closed (well, actually it's a race condition, it's not guaranteed)

我建议:

with open(images_dir+'/'+photo_name,'wb') as output:
    output.write(photo_file)
subprocess.check_call(['touch','-d','{}'.format(date_in),'{}'.format(images_dir+'/'+photo_name)])

所以当你调用 check_call

写得更好:

fullpath = os.path.join(images_dir,photo_name)
with open(fullpath ,'wb') as output:
    output.write(photo_file)
# we're outside the with block, note the de-indentation
subprocess.check_call(['touch','-d','{}'.format(date_in),fullpath])

这篇关于Python 中的系统 vs 调用 vs popen的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！