ZtoYi - 6 months ago 23

Python Question

I have some python code that looks like this:

`mat = [[3], [4], [4], [0], [1, 2]]`

nwalls = 5*[1]

for i in range(1,3):

nwalls = [sum(nwalls[k] for k in mat[j]) for j in range(5)]

# nwalls = [1, 2, 2, 1, 2]

I can't for the life of me unroll this into syntax without using list comprehension for me to understand. Please assist.

Answer

The direct translation would be

```
mat = [[3], [4], [4], [0], [1, 2]]
nwalls = 5*[1]
for i in range(1, 3):
_nwalls = []
for j in range(5):
tot = 0 # - sum
for k in mat[j]: # /
tot += nwalls[k] # /
_nwalls.append(tot)
nwalls = _nwalls
```

`(nwalls[k] for k in mat[j])`

it self is a generator, in python repl, you can check it as:

```
>>> y = (x for x in range(10))
>>> type(y)
<class 'generator'>
```

and `sum`

can take a generator, as `sum( (x for x in range(10)) )`

, and as PEP289 said

if a function call has a single positional argument, it can be a generator expression without extra parentheses, but in all other cases you have to parenthesize it.

so it looks like `sum(x for x in range(10))`