Get size of a file before downloading in Python

Question

I\'m downloading an entire directory from a web server. It works OK, but I can\'t figure how to get the file size before download to compare if it was updated on the server

Answer 1

@PabloG Regarding the local/server filesize difference

Following is high-level illustrative explanation of why it may occur:

The size on disk sometimes is different from the actual size of the data. It depends on the underlying file-system and how it operates on data. As you may have seen in Windows when formatting a flash drive you are asked to provice 'block/cluster size' and it varies [512b - 8kb]. When a file is written on the disk, it is storled in a 'sort-of linked list' of disk blocks. When a certain block is used to store part of a file, no other file contents will be stored in the same blok, so even if the chunk is no occupuing the entire block space, the block is rendered unusable by other files.

Example: When the filesystem is divided on 512b blocks, and we need to store 600b file, two blocks will be occupied. The first block will be fully utilized, while the second block will have only 88b utilized and the remaining (512-88)b will be unusable resulting in 'file-size-on-disk' being 1024b. This is why Windows has different notations for 'file size' and 'size on disk'.

NOTE: There are different pros & cons that come with smaller/bigger FS block, so do a better research before playing with your filesystem.

Answer 2

The size of the file is sent as the Content-Length header. Here is how to get it with urllib:

>>> site = urllib.urlopen("http://python.org")
>>> meta = site.info()
>>> print meta.getheaders("Content-Length")
['16535']
>>>

Answer 3

I have reproduced what you are seeing:

import urllib, os
link = "http://python.org"
print "opening url:", link
site = urllib.urlopen(link)
meta = site.info()
print "Content-Length:", meta.getheaders("Content-Length")[0]

f = open("out.txt", "r")
print "File on disk:",len(f.read())
f.close()


f = open("out.txt", "w")
f.write(site.read())
site.close()
f.close()

f = open("out.txt", "r")
print "File on disk after download:",len(f.read())
f.close()

print "os.stat().st_size returns:", os.stat("out.txt").st_size

Outputs this:

opening url: http://python.org
Content-Length: 16535
File on disk: 16535
File on disk after download: 16535
os.stat().st_size returns: 16861

What am I doing wrong here? Is os.stat().st_size not returning the correct size?

---

Edit: OK, I figured out what the problem was:

import urllib, os
link = "http://python.org"
print "opening url:", link
site = urllib.urlopen(link)
meta = site.info()
print "Content-Length:", meta.getheaders("Content-Length")[0]

f = open("out.txt", "rb")
print "File on disk:",len(f.read())
f.close()


f = open("out.txt", "wb")
f.write(site.read())
site.close()
f.close()

f = open("out.txt", "rb")
print "File on disk after download:",len(f.read())
f.close()

print "os.stat().st_size returns:", os.stat("out.txt").st_size

this outputs:

$ python test.py
opening url: http://python.org
Content-Length: 16535
File on disk: 16535
File on disk after download: 16535
os.stat().st_size returns: 16535

Make sure you are opening both files for binary read/write.

// open for binary write
open(filename, "wb")
// open for binary read
open(filename, "rb")

Answer 4

For a python3 (tested on 3.5) approach I'd recommend:

with urlopen(file_url) as in_file, open(local_file_address, 'wb') as out_file:
    print(in_file.getheader('Content-Length'))
    out_file.write(response.read())

Answer 5

In Python3:

>>> import urllib.request
>>> site = urllib.request.urlopen("http://python.org")
>>> print("FileSize: ", site.length)

Answer 6

Using the returned-urllib-object method info(), you can get various information on the retrived document. Example of grabbing the current Google logo:

>>> import urllib
>>> d = urllib.urlopen("http://www.google.co.uk/logos/olympics08_opening.gif")
>>> print d.info()

Content-Type: image/gif
Last-Modified: Thu, 07 Aug 2008 16:20:19 GMT  
Expires: Sun, 17 Jan 2038 19:14:07 GMT 
Cache-Control: public 
Date: Fri, 08 Aug 2008 13:40:41 GMT 
Server: gws 
Content-Length: 20172 
Connection: Close

It's a dict, so to get the size of the file, you do urllibobject.info()['Content-Length']

print f.info()['Content-Length']

And to get the size of the local file (for comparison), you can use the os.stat() command:

os.stat("/the/local/file.zip").st_size