Python or matplotlib limitation error

I wrote an algorithm using python and matplotlib that generates histograms from some text input data. When the number of data input is approx. greater than 15000, I get in the (append) line of my code:

mydata = []

for i in range(len(开发者_高级运维data)):
   mydata.append(string.atof(data[i]))

the error:

Traceback (most recent call last):
  File "get_histogram_picture.py", line 25, in <module>
    mydata.append(string.atof(data[i]))
  File "/usr/lib/python2.6/string.py", line 388, in atof
    return _float(s)
ValueError: invalid literal for float(): -a

can it be an error in python ? What is the solution ?

Thanks

---

That's a data parsing error:

>>> float("-a")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for float(): -a

Python data structure size if only limited by the available memory.