Does Python have an argc argument?

Question

I have written the same program (open text file and display contents) in C and C++. Now am doing the same in Python (on a Linux machine).

In the C programs I used the code:

if (argc != 2) {
    /* exit program */
}

Question: What is used in Python to check the number of arguments

#!/usr/bin/python
import sys
try:
    in_file = open(sys.argv[1], "r")
except:
    sys.exit("ERROR. Did you make a mistake in the spelling")
text = in_file.read()
print text
in_file.close()

Current output:

./python names.txt = Displays text file (correct)
./python nam = error message: stated from the sys.ext line (correct)
./python = error message: stated from the sys.ext line (wrong: want it to be a
separate error message stating *no file name input*)

Solution 1

In python a list knows its length, so you can just do len(sys.argv) to get the number of elements in argv.

Solution 2

I often use a quick-n-dirty trick to read a fixed number of arguments from the command-line:

[filename] = sys.argv[1:]

in_file = open(filename)   # Don't need the "r"

This will assign the one argument to filename and raise an exception if there isn't exactly one argument.

Solution 3

You're better off looking at argparse for argument parsing.

http://docs.python.org/dev/library/argparse.html

Just makes it easy, no need to do the heavy lifting yourself.

Comments

[jww]

Possible duplicate of python and sys.argv

[ThorSummoner]

From a object orientation perspective, I can understand why argc should be coupled to argv. If these were two objects, they could be mutated interdependently of each other. It makes sense for argc to be an attribute of argv (and not of sys), I just am not sure len is the defacto attribute name i would use. argv.__len__ is critical for mechanical reasons, and since its there it doesn't make a ton of sense to give it another name. I would probably have favored a sys proxy descriptor like sys.argc = property(lambda self: len(self.argv)).__get__(sys, sys.__class__) though

[Teo]

import sys # count the arguments arguments = len(sys.argv) - 1 print ("the script is called with %i arguments" % (arguments))

[DoomGoober]

I like "better to ask forgiveness than permission" for most cases in Python. In the case of command line args though, an arguably better user experience would be to show the command line arguments that are required for the Python script if the wrong number of arguments are present.

[Cees Timmerman]

@DoomGoober "supplied filename" is the argument it expects.