How to properly get coordinates from gpx file in python

I try to extract coordinates from gpx file and it returns empty list. I used xml.etree and have any idea why it is empty, because d.attrib return proper values.

import xml.etree.ElementTree as ET
f = ('St_Louis_Zoo_sample.gpx')
p = ET.parse(f)
root = p.getroot()
lats=[]
lons=[]

for d in root:
    if d.tag == 'wpt':       
        y = d.attrib['lat']

        x = d.attrib['lon']
        lats.append(y)
        lons.append(x)

Here it is a gpx file

  St Louis Zoo sample

  This self guided,GPS enabled tour of the world famous St. Louis Zoo, has 85 points of interest. Narratives in english,explaining each exhibit and provides guidance to zoo facilities.This audio tour guide can enhance your next visit.
  
    wizardone, using GeoTours
    
      St Louis Zoo sample
    
  
  
    St Louis Zoo sample
  

  2008-02-26T19:49:13
  Audio tour guide
St.Louis Mo.
Zoo
Forest Park
Animals





Asian Elephant
elephant

Waypoint

        
            15.24
            SymbolAndName
        





Bactrian Camel
camel

Waypoint

        
            15.24

            SymbolAndName
        




Black Rhinoceros
black rhino

Waypoint


        
            15.24
            SymbolAndName
        




Cafe Restroom

cafe restroom

Waypoint

        
            15.24
            SymbolAndName
        





Polar Bear
polar bear

Waypoint

        
            15.24
            SymbolAndName

        




California Sea Lion
sea lion

Waypoint


        
            15.24
            SymbolAndName
        




Cheetah
cheetah


Waypoint

        
            15.24
            SymbolAndName
        





Grizzley Bear
grizzly bear

Waypoint

        
            15.24
            SymbolAndName
        





Jaguar
jaguar

Waypoint

        

            15.24
            SymbolAndName
        




Lion
African lions live in a number of different habitats: grassy plains, open woodlands, semi-desert areas, even high mountains.


Waypoint

        
            15.24
            SymbolAndName
        

Answer

GPX files use XML namespaces (look at Parsing XML with namespace in Python via 'ElementTree' or Read GPX using Python ElementTree.register_namespace? for example).

The namespace is {http://www.topografix.com/GPX/1/1}

import xml.etree.ElementTree as ET
tree = ET.parse("St_Louis_Zoo_sample.gpx")
for elem in tree.findall("{http://www.topografix.com/GPX/1/1}wpt"):
    print elem, elem.attrib['lon'], elem.attrib['lat']

 -90.29408 38.63473
 -90.28679 38.63368
 -90.29323 38.63408

 -90.29019 38.63533
 -90.28976 38.63677
 -90.28948 38.63496
 -90.29458 38.63421
 -90.29083 38.63633
 -90.28715 38.63395
 -90.28769 38.63347

With ElementTree (xml.etree), you can define the namespace before

 namespace = {"gpx": "http://www.topografix.com/GPX/1/1"} 

 for elem in tree.findall('gpx:wpt', namespace):
        print elem.attrib['lon'],  elem.attrib['lat']
 -90.29408 38.63473
 -90.28679 38.63368
 -90.29323 38.63408
 -90.29019 38.63533
 -90.28976 38.63677
 -90.28948 38.63496
 -90.29458 38.63421
 -90.29083 38.63633

 -90.28715 38.63395
 -90.28769 38.63347

As with lxml

from lxml import etree
NSMAP = {"gpx": "http://www.topografix.com/GPX/1/1"}
tree = etree.parse("St_Louis_Zoo_sample.gpx")
for elem in tree.findall("gpx:wpt", namespaces=NSMAP):
     print elem.attrib['lon'], elem.attrib['lat']
-90.29408 38.63473

-90.28679 38.63368
-90.29323 38.63408
-90.29019 38.63533
....