{"id":4555,"date":"2015-04-19T23:26:36","date_gmt":"2015-04-19T15:26:36","guid":{"rendered":"http:\/\/rmohan.com\/?p=4555"},"modified":"2015-04-19T23:37:29","modified_gmt":"2015-04-19T15:37:29","slug":"python-if-statement","status":"publish","type":"post","link":"https:\/\/mohan.sg\/?p=4555","title":{"rendered":"Python file and DIRECTORY"},"content":{"rendered":"
import<\/span> os\r\n<\/span>import<\/span> os<\/span>.<\/span>path\r\n\r\nPATH<\/span>=<\/span>'.\/file.txt'<\/span>\r\n\r\nif<\/span> os<\/span>.<\/span>path<\/span>.<\/span>isfile<\/span>(<\/span>PATH<\/span>)<\/span> and<\/span> os<\/span>.<\/span>access<\/span>(<\/span>PATH<\/span>,<\/span> os<\/span>.<\/span>R_OK<\/span>):<\/span>\r\n    print<\/span> \"File exists and is readable\"<\/span>\r\nelse<\/span>:<\/span>\r\n    print<\/span> \"Either file is missing or is not readable\"\r\n\r\n\r\n<\/span><\/code><\/pre>\n
import<\/span> os\r\nfname <\/span>=<\/span> \"foo.txt\"<\/span>\r\nif<\/span> os<\/span>.<\/span>path<\/span>.<\/span>isfile<\/span>(<\/span>fname<\/span>):<\/span>\r\n    print<\/span> \"file does exist at this time\"<\/span>\r\nelse<\/span>:<\/span>\r\n    print<\/span> \"no such file\"<\/span><\/code><\/pre>\n
    \n
  1. Try opening the file:Opening the file will always verify the existence of the file. You can make a function just like so:\n
    def<\/span> File_Existence<\/span>(<\/span>filepath<\/span>):<\/span>\r\n    f <\/span>=<\/span> open<\/span>(<\/span>filepath<\/span>)<\/span>\r\n    return<\/span> True<\/span> <\/code><\/pre>\n
    If it’s False, it will stop execution with an unhanded IOError or OSError in later versions of python. To catch the exception, you have to use a try except clause. Of course, you can always use a\u00a0<\/span>try<\/code>\u00a0<\/span>except<\/code>\u00a0<\/span>statement like so (Thanks to\u00a0<\/span>hsandt<\/a>\u00a0<\/span>for making me think):<\/p>\n
    def<\/span> File_Existence<\/span>(<\/span>filepath<\/span>):<\/span>\r\n    try<\/span>:<\/span>\r\n        f <\/span>=<\/span> open<\/span>(<\/span>filepath<\/span>)<\/span>\r\n    except<\/span> IOError<\/span>,<\/span> OSError<\/span>:<\/span> # Note OSError is for later versions of python<\/span>\r\n        return<\/span> False<\/span>\r\n\r\n    return<\/span> True<\/span><\/code><\/pre>\n<\/li>\n
  2. Use\u00a0<\/span>os.path.exists(path)<\/code>:This will check the existence of what you specify. However, it checks for files\u00a0<\/span>and<\/em>\u00a0<\/span>directories so beware about how you use it.\n
    import<\/span> os<\/span>.<\/span>path\r\n<\/span>>>><\/span> os<\/span>.<\/span>path<\/span>.<\/span>exists<\/span>(<\/span>\"this\/is\/a\/directory\"<\/span>)<\/span>\r\nTrue<\/span>\r\n>>><\/span> os<\/span>.<\/span>path<\/span>.<\/span>exists<\/span>(<\/span>\"this\/is\/a\/file.txt\"<\/span>)<\/span>\r\nTrue<\/span>\r\n>>><\/span> os<\/span>.<\/span>path<\/span>.<\/span>exists<\/span>(<\/span>\"not\/a\/directory\"<\/span>)<\/span>\r\nFalse<\/span><\/code><\/pre>\n<\/li>\n
  3. Use\u00a0<\/span>os.access(path, mode)<\/code>:This will check whether you have access to the file. It will check for permissions. Based on the os.py documentation, typing in\u00a0<\/span>os.F_OK<\/code>, will check the existence of the path. However, using this will create a security hole, as someone can attack your file using the time between checking the permissions and opening the file. You should instead go directly to opening the file instead of checking its permissions. (EAFP<\/a>\u00a0<\/span>vs\u00a0<\/span>LBYP<\/a>). If you’re not going to open the file afterwards, and only checking its existence, then you can use this.Anyways, here:\n
    >>><\/span> import<\/span> os\r\n<\/span>>>><\/span> os<\/span>.<\/span>access<\/span>(<\/span>\"\/is\/a\/file.txt\"<\/span>,<\/span> os<\/span>.<\/span>F_OK<\/span>)<\/span>\r\nTrue<\/span><\/code><\/pre>\n<\/li>\n<\/ol>\n
    \u00a0\r\n\r\n\r\nDirectory \r\n\r\n\r\n\r\n<\/span><\/code><\/pre>\n
    import<\/span> os\r\n<\/span>print<\/span>(<\/span>os<\/span>.<\/span>path<\/span>.<\/span>isdir<\/span>(<\/span>\"\/home\/el\"<\/span>))<\/span>\r\nprint<\/span>(<\/span>os<\/span>.<\/span>path<\/span>.<\/span>exists<\/span>(<\/span>\"\/home\/el\/myfile.txt\"<\/span>))\r\n\r\n<\/span><\/code><\/pre>\n
    os provides you with a lot of these capabilities:<\/p>\n
    import<\/span> os\r\nos<\/span>.<\/span>path<\/span>.<\/span>isdir<\/span>(<\/span>dir_in<\/span>)<\/span> #True\/False: check if this is a directory<\/span>\r\nos<\/span>.<\/span>listdir<\/span>(<\/span>dir_in<\/span>)<\/span>    #gets you a list of all files and directories under dir_in<\/span><\/code><\/pre>\n
    \u00a0<\/span><\/code><\/pre>\n
    \u00a0\r\n\r\nMake directory on python \r\n\r\n<\/span><\/code><\/pre>\n
    import<\/span> os\r\n\r\ndirname <\/span>=<\/span> 'create\/me'<\/span>\r\n\r\ntry<\/span>:<\/span>\r\n    os<\/span>.<\/span>makedirs<\/span>(<\/span>dirname<\/span>)<\/span>\r\nexcept<\/span> OSError<\/span>:<\/span>\r\n    if<\/span> os<\/span>.<\/span>path<\/span>.<\/span>exists<\/span>(<\/span>dirname<\/span>):<\/span>\r\n        # We are nearly safe<\/span>\r\n        pass<\/span>\r\n    else<\/span>:<\/span>\r\n        # There was an error on creation, so make sure we know about it<\/span>\r\n        raise<\/span><\/code><\/pre>\n
    \u00a0\r\n\r\n<\/span><\/code><\/pre>\n
    \n
    How To Check If A File Exists<\/h1>\n<\/header>\n
    \n
    If you find yourself doing any kind of disk-based I\/O with Python, you’ll undoubtedly come across the need to verify that a file exists before continuing to read\/write against it. There are multiple ways to do this but also some things you should watch for.<\/p>\n
    You may initially discover the\u00a0<\/span>os.path.exists()<\/strong>\u00a0<\/span>method, but keep in mind that this will yield True for both files\u00a0<\/span>and<\/em>\u00a0<\/span>directories. This lack of specificity could easily introduce bugs and data loss if not expected.<\/p>\n
    \n
    #Returns true for directories, not just files<\/span>\r\n>>><\/span> print<\/span> os<\/span>.<\/span>path<\/span>.<\/span>exists<\/span>(<\/span>'\/this\/is\/a\/dir'<\/span>)<\/span>\r\nTrue<\/span>\r\n<\/pre>\n<\/div>\n
    If you want to zero-in on just files, stick to\u00a0<\/span>os.path.isfile()<\/strong>\u00a0<\/span>method. Remember that something could happen to a file in the time between checking that it exists and actually preforming read\/write operations against it. This approach will not lock the file in any way and therefore your code can become vulnerable to “time of check to time of use” (TOCTTOU) bugs.<\/p>\n
    \n
    #Returns true for directories, not just files<\/span>\r\nif<\/span> os<\/span>.<\/span>path<\/span>.<\/span>isfile<\/span>(<\/span>'my_settings.dat'<\/span>):<\/span>\r\n    #...<\/span>\r\n    with<\/span> open<\/span>(<\/span>'my_settings.dat'<\/span>)<\/span> as<\/span> file<\/span>:<\/span>\r\n       pass<\/span>  #Potential for unhandled exception<\/span>\r\nTrue<\/span>\r\n<\/pre>\n<\/div>\n
    Yes, that’s a real thing.\u00a0<\/span>Wikipedia has an article on TOCTTOU bugs<\/a>, along with an example of how symlinks can be used by an attacker. I encourage you to read it.<\/p>\n
    Raising exceptions is considered to be an acceptable, and Pythonic, approach for flow control in your program. Consider handling missing files with IOErrors, rather than\u00a0<\/span>if<\/em>\u00a0<\/span>statements. In this situation, an IOError exception will be raised if the file exists but the user does not have read permissions.<\/p>\n
    \n
    try<\/span>:<\/span>\r\n    with<\/span> open<\/span>(<\/span>'my_settings.dat'<\/span>)<\/span> as<\/span> file<\/span>:<\/span>\r\n        pass<\/span>\r\nexcept<\/span> IOError<\/span> as<\/span> e<\/span>:<\/span>\r\n    print<\/span> \"Unable to open file\"<\/span> #Does not exist OR no read permissions<\/span>\r\n<\/pre>\n<\/div>\n<\/div>\n

    \r\n<\/span><\/code><\/pre>\n
    Check If a File Exists and Then Delete It in Python<\/h2>\n
    \u00a0\r\n#!\/usr\/bin\/python\r\nimport os\r\n\u00a0\r\n## get input ##\r\nfilename=raw_input(\"Type file name to remove: \")\r\n\u00a0\r\n## delete only if file exists ##\r\nif os.path.exists(filename):\r\n    os.remove(filename)\r\nelse:\r\n    print(\"Sorry, I can not remove %s file.\" % filename)\r\n<\/pre>\n
     <\/p>\n
    A Better Option To Delete A File In Python<\/h2>\n
    The following code gives an error information when it can not delete the given file name:<\/p>\n
    \u00a0\r\n#!\/usr\/bin\/python\r\nimport os\r\n\u00a0\r\n## get input ##\r\nfilename=raw_input(\"Type file name to remove: \")\r\n\u00a0\r\n## check if a file exists on disk ##\r\n## if exists, delete it else show message on screen ##\r\nif os.path.exists(filename):\r\n\ttry:\r\n\t\tos.remove(filename)\r\n\texcept OSError, e:\r\n\t\tprint (\"Error: %s - %s.\" % (e.filename,e.strerror))\r\nelse:\r\n\tprint(\"Sorry, I can not find %s file.\" % filename)\r\n<\/pre>\n
    Sample outputs:<\/p>\n","protected":false},"excerpt":{"rendered":"
    import os import os.path PATH=’.\/file.txt’ if os.path.isfile(PATH) and os.access(PATH, os.R_OK): print “File exists and is readable” else: print “Either file is missing or is not readable” import os fname = “foo.txt” if os.path.isfile(fname): print “file does exist at this time” else: print “no such file” Try opening the file:Opening the file will always verify the […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[74],"tags":[],"_links":{"self":[{"href":"https:\/\/mohan.sg\/index.php?rest_route=\/wp\/v2\/posts\/4555"}],"collection":[{"href":"https:\/\/mohan.sg\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/mohan.sg\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/mohan.sg\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/mohan.sg\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=4555"}],"version-history":[{"count":7,"href":"https:\/\/mohan.sg\/index.php?rest_route=\/wp\/v2\/posts\/4555\/revisions"}],"predecessor-version":[{"id":4562,"href":"https:\/\/mohan.sg\/index.php?rest_route=\/wp\/v2\/posts\/4555\/revisions\/4562"}],"wp:attachment":[{"href":"https:\/\/mohan.sg\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4555"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/mohan.sg\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4555"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/mohan.sg\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4555"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}