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unzip: perform the opposite of zip
zip
>>> a = [1, 2, 3] >>> b = ["one", "two", "three"] >>> zip(a, b) <zip object at 0x7fd30310b508> >>> list(zip(a, b)) [(1, 'one'), (2, 'two'), (3, 'three')]
unzip
How to perform the opposite of zip? That is, we have [(1, 'one'), (2, 'two'), (3, 'three')], and we want to get back [1, 2, 3] and ["one", "two", "three"].
>>> li
[(1, 'one'), (2, 'two'), (3, 'three')]
>>> a, b = zip(*li)
>>> a
(1, 2, 3)
>>> b
('one', 'two', 'three')
Notice that the results are tuples.
More info here.
working with zip files
Problem
In a project of mine I had to deal with folders, where a folder can contain several thousands of small text files. I kept this project on Dropbox, so I could use it on all my machines. However, Dropbox is quite slow when trying to synchronize several thousand files. So I decided to put files in a folder into a zip file.
So the question is: how to deal with zip files? How to do basic operations with them: create zip, delete from zip, list zip, add to zip, move to zip, extract from zip, etc.
Solution
In this project of mine I used the external zip command as well as the zipfile package from the stdlib. Let’s see both of them.
Manipulating zip files from the command-line
Let’s see some examples. Compress every .json file in the current directory except the desc.json file:
zip -9 files.zip *.json -x desc.json
The switch “-9” gives the best compression, files.zip is the output, and “-x” is short for “--exclude“. From Python you can call it as an external command with os.system() for instance.
The previous example creates a zip file and leaves the original files. Now let’s move files into a zip file (and delete the original files when they were added successfully to the archive):
zip -9 -m files.zip *.json -x desc.json
Delete a file from an archive:
zip -d files.zip desc.json
It will delete desc.json from the zip file.
List the content of a zip file:
zipinfo files.zip
Add a file to the archive:
zip -g files.zip new.json
Where “-g” means: grow.
Extract just one file from a zip file:
# basic: unzip files.zip this.json # extract to a specific folder: unzip files.zip this.json -d /extract/here/
It will extract this.json from the archive.
Read the content of a zip file in Python
OK, say we have a zip file that contains some files. How to get the filenames? How to read them? I found some nice examples here.
List the file names in a zip file:
import zipfile
zfile = zipfile.ZipFile("files.zip", "r")
for name in zfile.namelist():
print(name)
Read files in a zip file:
import zipfile
zfile = zipfile.ZipFile("files.zip", "r")
for name in zfile.namelist():
data = zfile.read(name)
print(data)
Links
- The zipfile module at effbot.org.
Python equivalent of Java .jar files
Problem
In Java, you can distribute your project in JAR format. It is essentially a ZIP file with some metadata. The project can be launched easily:
$ java -jar project.jar
What is its Python equivalent? How to distribute a Python project (with several modules and packages) in a single file?
Solution
The following is based on this post, written by bheklilr. Thanks for the tip.
Let’s see the following project structure:
MyApp/
MyApp.py <--- Main script
alibrary/
__init__.py
alibrary.py
errors.py
anotherlib/
__init__.py
another.py
errors.py
configs/
config.json
logging.json
Rename the main script to __main__.py and compress the project to a zip file. The extension can be .egg:
myapp.egg/ <--- technically, it's just a zip file
__main__.py <--- Renamed from MyApp.py
alibrary/
__init__.py
alibrary.py
errors.py
anotherlib/
__init__.py
another.py
errors.py
configs/
config.json
logging.json
How to zip it? Enter the project directory (MyApp/) and use this command:
zip -r ../myapp.egg .
Now you can launch the .egg file just like you launch a Java .jar file:
$ python myapp.egg
You can also use command-line arguments that are passed to __main__.py.
Hamming distance
The Hamming distance is defined between two strings of equal length. It measures the number of positions with mismatching characters.
Example: the Hamming distance between “toned” and “roses” is 3.
#!/usr/bin/env python
def hamming_distance(s1, s2):
assert len(s1) == len(s2)
return sum(ch1 != ch2 for ch1, ch2 in zip(s1, s2))
if __name__=="__main__":
a = 'toned'
b = 'roses'
print hamming_distance(a, b) # 3
If you need the number of matching character positions:
#!/usr/bin/env python
def similarity(s1, s2):
assert len(s1) == len(s2)
return sum(ch1 == ch2 for ch1, ch2 in zip(s1, s2))
if __name__=="__main__":
a = 'toned'
b = 'roses'
print similarity(a, b) # 2
Actually this is equal to len(s1) - hamming_distance(s1, s2). Remember, len(s1) == len(s2).
More info on zip() here.
Python Adventures 