How to Print a Generator Expression

How to print a generator expression?

Quick answer:

Doing list() around a generator expression is (almost) exactly equivalent to having [] brackets around it. So yeah, you can do

>>> list((x for x in string.letters if x in (y for y in "BigMan on campus")))

But you can just as well do

>>> [x for x in string.letters if x in (y for y in "BigMan on campus")]

Yes, that will turn the generator expression into a list comprehension. It's the same thing and calling list() on it. So the way to make a generator expression into a list is to put brackets around it.

Detailed explanation:

A generator expression is a "naked" for expression. Like so:

x*x for x in range(10)

Now, you can't stick that on a line by itself, you'll get a syntax error. But you can put parenthesis around it.

>>> (x*x for x in range(10))
<generator object <genexpr> at 0xb7485464>

This is sometimes called a generator comprehension, although I think the official name still is generator expression, there isn't really any difference, the parenthesis are only there to make the syntax valid. You do not need them if you are passing it in as the only parameter to a function for example:

>>> sorted(x*x for x in range(10))
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

Basically all the other comprehensions available in Python 3 and Python 2.7 is just syntactic sugar around a generator expression. Set comprehensions:

>>> {x*x for x in range(10)}
{0, 1, 4, 81, 64, 9, 16, 49, 25, 36}

>>> set(x*x for x in range(10))
{0, 1, 4, 81, 64, 9, 16, 49, 25, 36}

Dict comprehensions:

>>> dict((x, x*x) for x in range(10))
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}

>>> {x: x*x for x in range(10)}
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}

And list comprehensions under Python 3:

>>> list(x*x for x in range(10))
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

>>> [x*x for x in range(10)]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

Under Python 2, list comprehensions is not just syntactic sugar. But the only difference is that x will under Python 2 leak into the namespace.

>>> x
9

While under Python 3 you'll get

>>> x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'x' is not defined

This means that the best way to get a nice printout of the content of your generator expression in Python is to make a list comprehension out of it! However, this will obviously not work if you already have a generator object. Doing that will just make a list of one generator:

>>> foo = (x*x for x in range(10))
>>> [foo]
[<generator object <genexpr> at 0xb7559504>]

In that case you will need to call list():

>>> list(foo)
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

Although this works, but is kinda stupid:

>>> [x for x in foo]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

How to print the content of the generator?

It's because you passed a generator to a function and that's what __repr__ method of this generator returns. If you want to print what it would generate, you can use:

print(*N, sep='\n') # * will unpack the generator

or

print('\n'.join(map(str, N)))

Note that once you retrieve the generator's output to print it, the generator is exhausted - trying to iterate over it again will produce no items.

How do I output the result of a generator expression in CMake?

How do I print the result of the evaluated generator expression during configuration?

You cannot. Generator expressions are intended for things, which are not exactly known at configuration stage: they depend on build type, which, in case of multiconfiguration generators, becomes known only at the build stage.

You may, however, save a value of the generator expression into the file, but the file will be written only at the end of the configuration stage:

file(GENERATE OUTPUT <filename> CONTENT <string-with-generator-expression>)

More detailed description of file(GENERATOR) see in documentation.

How to print unicode from a generator expression in python?

Unicode is a character to bytes encoding, not escape sequences. Python 3 strings are Unicode. To return the character that corresponds to a Unicode code point use chr :

chr(i)
Return the string representing a character whose Unicode code point is the integer i. For example, chr(97) returns the string 'a', while chr(8364) returns the string '€'. This is the inverse of ord().

The valid range for the argument is from 0 through 1,114,111 (0x10FFFF in base 16). ValueError will be raised if i is outside that range.

To generate the characters between 2663 and 2670:

>>> [chr(x) for x  in range(2663,2670)]
['੧', '੨', '੩', '੪', '੫', '੬', '੭']

Escape sequences use hexadecimal notation though. 0x2663 is 9827 in decimal, and 0x2670 becomes 9840.

>>> [chr(x) for x  in range(9827,9840)]
['♣', '♤', '♥', '♦', '♧', '♨', '♩', '♪', '♫', '♬', '♭', '♮', '♯']

You can use also use hex numeric literals:

>>> [chr(x) for x  in range(0x2663,0x2670)]
['♣', '♤', '♥', '♦', '♧', '♨', '♩', '♪', '♫', '♬', '♭', '♮', '♯']

or, to use exactly the same logic as the question

>>> [chr(0x2600 + x) for x  in range(0x63,0x70)]
['♣', '♤', '♥', '♦', '♧', '♨', '♩', '♪', '♫', '♬', '♭', '♮', '♯']

The reason the original code doesn't work is that escape sequences are used to represent a single character in a string when we can't or don't want to type the character itself. The interpreter or compiler replaces them with the corresponding character immediatelly. The string \\u26 is an escaped \ followed by u, 2 and 6:

>>> len('\\u26')
4

Why generator function does not work with the next call?

Each call to getPositiveCount() creates a brand new generator. You are thinking of this:

gen = getPositiveCount()
print(next(gen))
print(next(gen))
print(next(gen))

---

Related Topics