import json
a = [1.2, 2.9, 7.4]
str = "t".join(format(x,"10.3f") for x in a)
print json.dumps(str)

"     1.200t     2.900t     7.400"

"1.200t2.900t7.400"

s = "t".join(format(x, "10.3f").strip() for x in a)

import json
a = [1.2, 2.9, 7.4]
s = "t".join(format(x, "10.3f").strip() for x in a)
print json.dumps(s) # "1.200t2.900t7.400"

str = "t".join(format(x,"0.3f") for x in a)