Python习语用于赋值的布尔评估

Question

I have the following C program as an example of what I would like to be able to do in python:

foo@foo:~/$ cat test.c 
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

bool get_false(){
    return false;
}

bool get_true(){
    return true;
}

void main(int argc, char* argv[]){

    bool x, y;

    if ( x = get_false() ){
        printf("Whiskey. Tango. Foxtrot.\n");
    }   

    if ( y = get_true() ){
        printf("Nothing to see here, keep moving.\n");
    }   
}
foo@foo:~/$ gcc test.c -o test
test.c: In function ‘main’:
test.c:13: warning: return type of ‘main’ is not ‘int’
foo@foo:~/$ ./test 
Nothing to see here, keep moving.
foo@foo:~/$ 

In python, the only way I know how to do this is:

foo@foo:~/$ cat test.py
def get_false():
    return False

def get_true():
    return True

if __name__ == '__main__':
    x = get_false()
    if x:
        print "Whiskey. Tango. Foxtrot."

    y = get_true()
    if y:
        print "Nothing to see here, keep moving."

    #if (z = get_false()):
    #    print "Uncommenting this will give me a syntax error."

    #if (a = get_false()) == False:
    #    print "This doesn't work either...also invalid syntax."
foo@foo:~/$ python test.py
Nothing to see here, keep moving.

Why? Because I'd like to be able to say:

if not (x=get_false()): x={}

Basically I'm working around a bad API where the type returned is either a dict when data is available, or False. Yes, a valid answer would be to return consistent types and to use Exceptions instead of False for a failure mode indicator. I can't change the underlying API though, and I run into this pattern quite a bit in environments like Python with dynamic typing (read: without strict typing for function/method interfaces).

Any suggestions on how to reduce the if/else overhead?

Answer 1

You can use

x = get_false() or {}

Should get_false() return a False value, Python will return the second operand of or .

See section 5.10 of the Python reference manual. (It's been there since at least Python 2.0 ).

Answer 2

You are compounding an inconvenient API with repetitive bad patch up code thus compounding the problem that you seek to avoid.

def wrapper():
    x = get_false()
    if not x:
        x = dict()
    return x

Then your code isn't littered with hard-to-read ternary (or ternary-like) operations and you can change wrapper to raise exceptions if you find that more suitable.

What you can't do is have an assignment as a conditional as in C; Python doesn't do that.

Answer 3

You could use the Python ternary operator for this:

>>> data=False    # could be data=readyourapi()
>>> x=data if data else {}
>>> x
{}