>>> import arcpy
import os
from arcpy import env
env.workspace = r"D:Julia_Tprojekterste_schrittedritter_tag_pythontestdaten"

#folder to list
ascFileList = arcpy.ListFiles("*.asc")

for ascFile in ascFileList:
# geoprocessing steps
ascFileName = os.path.splitext(ascFile)[0]
# define the output file
rastFile = ascFileName + "_output.img"
# run the tool
arcpy.ASCIIToRaster_conversion(ascFile,rastFile, "INTEGER")
Runtime error  Traceback (most recent call last):   File "<string>", line 15, in      <module>   File "c:programmearcgisdesktop10.2arcpyarcpyconversion.py", line 2193, in    ASCIIToRaster     raise e ExecuteError: ERROR 000865: Eingabe-ASCII-Raster-Datei:    2001002_cloud_free.asc ist nicht vorhanden.