I am reading "Learn Python the Hard Way" and was confused by the "script" part of the second line.
from sys import argv
script, filename = argv
From what I understand, the second line says: script
and filename
comprise argv
.
I tried running my code without the "script" part and it worked just fine. I'm not sure what the purpose of it is.
Short answer import the argv list from the sys package. argv contains parameters passed in to python on the command line.
argv is a list containing the arguments passed to the python interpreter when called from a command line.
sys. argv is the list of commandline arguments passed to the Python program. argv represents all the items that come along via the command line input, it's basically an array holding the command line arguments of our program. Don't forget that the counting starts at zero (0) not one (1).
The first element of the array, argv[0] , is a pointer to the character array that contains the program name or invocation name of the program that is being run from the command line. argv[1] indicates the first argument passed to the program, argv[2] the second argument, and so on.
Generally, the first argument to a command-line executable is the script name, and the rest are the expected arguments.
Here, argv
is a list that is expected to contain two values: the script name and an argument. Using Python's unpacking notation, you can write
script = argv[0]
filename = argv[1]
as
script, filename = argv
while also throwing errors if there are an unexpected number of arguments (like one or three). This can be a good idea, depending on one's code, because it also ensures that there are no unexpected arguments.
However, the following code will not result in filename
actually containing the filename:
filename = argv
This is because filename
is now the argument list. To illustrate:
script, filename = argv
print("Script:", script) # Prints script name
print("Filename:", filename) # Prints the first argument
filename = argv
print("Filname:", filename) # Prints something like ["my-script.py", "my-file.txt"]
Others have explained what is script
, but the python statement is called unpacking and is usually applied to tuples or sequences.
It is a shortcut way of assigning a variable for each value that is in the tuple (or sequence) to the right of the =
sign.
It is not something specific to argv
:
>>> a,b = ('Hello','World')
>>> a
'Hello'
>>> b
'World'
One thing to keep in mind is that the number of variables on the left side must match the number of items in the sequence on the right, else you get:
>>> a,b,c = ('Hello','World')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: need more than 2 values to unpack
>>> a,b = ('Hello','World','!')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: too many values to unpack
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