dynamodb2: ProjectionExpression with ExpressionAttributeNames breaks with prefixes

[beheh]

I'm trying to query a table using moto's dynamodb2 backend on moto==1.3.7. Specifically, I'm querying my table with the following parameters:

{
  "ProjectionExpression": "#1, #2, #3, #4, #5, #6, #7, #8, #9, #10, #11, #12, #13, #14, #15",
  "ExpressionAttributeNames": {"#0": "foo", "#1": "bar", "#2": "baz", "#3": "baf", "#4": "puf", "#5": "par", "#6": "paz", "#7": "pauz", "#8": "pop", "#9": "plop", "#10": "plip", "#11": "plap", "#12": "plup", "#13": "bla", "#14": "ble", "#15": "bli"},
  ...
}

Note how #1 is a prefix of #10, #11, #12, #13, #14 and #15.

I'd expect moto to treat this like the following ProjectionExpression:

"foo, bar, baz, baf, puf, par, paz, pauz, pop, plop, plip, plap, plup, bla, ble, bli"

What happens instead:

"foo, bar, baz, baf, puf, par, paz, pauz, pop, plop, plip, foo1, foo2, foo3, foo4, foo5"

This is due to the following code in moto/dynamodb2/responses.py:

https://github.com/spulec/moto/blob/8cb4db1896521af99ff295b028f604c6ab599401/moto/dynamodb2/responses.py#L405-L409

Note how projection_expression.replace does a single string replacement, which means #1 will match the first two characters of #10, #11 and so on. A more graceful variant of that code would probably use the expressions after splitting to replace them one by one and reassembling to reassemble projection_expression.

All in all this results in an incomplete response missing the attributes with the AttributeName #10 and above. I've tested this against AWS and the error does not occur there.

I ran over this when using pynamodb, as it automatically creates this numerically indexed ProjectionExpression based on a list of attributes to get.

[beheh]

A very quick improvement of the bugged snippet from moto above might look like this:

        if projection_expression and expression_attribute_names:
            expressions = [x.strip() for x in projection_expression.split(',')]
            projection_expression = None
            for expression in expressions:
                if projection_expression is not None:
                    projection_expression = projection_expression + ", "
                else:
                    projection_expression = ""
                if expression in expression_attribute_names:
                    projection_expression = projection_expression + expression_attribute_names[expression]
                else:
                    projection_expression = projection_expression + expression

[spulec]

Thanks. A pull request would be very welcome!

[joolean]

As requested! #2412

[spulec]

Closed with #2412