我为'str'和'str'获取了不受支持的操作数类型，但我无法找到它。我读了很多问题，但还是不明白

Question

我试图在ArcGIS中构建一个工具，用于计算陆地卫星8波段（b8-b4）/（b8+b4）。我在arcmap中构建了模型，然后作为脚本导出。它运行一个模型，但作为一个工具。我在最后一行计算频带时出错

# Import arcpy module
import arcpy

# Script arguments
band8_input = arcpy.GetParameterAsText(0)
if band8_input == '#' or not band8_input:
    band8_input = "band8_input" # provide a default value if unspecified

band4_input = arcpy.GetParameterAsText(1)
if band4_input == '#' or not band4_input:
    band4_input = "band4_input" # provide a default value if unspecified

RGB_input = arcpy.GetParameterAsText(2)
if RGB_input == '#' or not RGB_input:
    RGB_input = "RGB_input" # provide a default value if unspecified

clip_polygon = arcpy.GetParameterAsText(3)
if clip_polygon == '#' or not clip_polygon:
    clip_polygon = "clip_polygon" # provide a default value if unspecified

# Local variables:
RGB_clp = "C:\\egm722\\RGB_clp"
RGB_clp_prj = RGB_clp
b8_clp = "C:\\egm722\\b8_clp"
b8_clp_prj = b8_clp
b4_clp = "C:\\egm722\\b4_clp"
b4_clp_prj = b4_clp
NVDI = "C:\\egm722\\NVDI.img"
NVDI_prj = NVDI

# Process: Clip_RGB
arcpy.Clip_management(RGB_input, "321255.095287406 5732480.81845376 327031.683798733 5736711.6778568", RGB_clp, clip_polygon, "256", "NONE", "NO_MAINTAIN_EXTENT")

# Process: Prj_RGB
arcpy.DefineProjection_management(RGB_clp, "GEOGCS['GCS_WGS_1984',DATUM['D_WGS_1984',SPHEROID['WGS_1984',6378137.0,298.257223563]],PRIMEM['Greenwich',0.0],UNIT['Degree',0.0174532925199433]]")

# Process: Clip_b8
arcpy.Clip_management(band8_input, "321255.095287406 5732480.81845376 327031.683798733 5736711.6778568", b8_clp, clip_polygon, "-32768", "NONE", "NO_MAINTAIN_EXTENT")

# Process: Prj_b8
arcpy.DefineProjection_management(b8_clp, "GEOGCS['GCS_WGS_1984',DATUM['D_WGS_1984',SPHEROID['WGS_1984',6378137.0,298.257223563]],PRIMEM['Greenwich',0.0],UNIT['Degree',0.0174532925199433]]")

# Process: Clip_b4
arcpy.Clip_management(band4_input, "321255.095287406 5732480.81845376 327031.683798733 5736711.6778568", b4_clp, clip_polygon, "-32768", "NONE", "NO_MAINTAIN_EXTENT")

# Process: Prj_b4
arcpy.DefineProjection_management(b4_clp, "GEOGCS['GCS_WGS_1984',DATUM['D_WGS_1984',SPHEROID['WGS_1984',6378137.0,298.257223563]],PRIMEM['Greenwich',0.0],UNIT['Degree',0.0174532925199433]]")

# Process: Calc_NVDI
arcpy.gp.RasterCalculator_sa("Float(\"%b8_clp_prj%\" - \"%b4_clp_prj%\") / (\"%b8_clp_prj%\" + \"%b4_clp_prj%\")", NVDI)

# Process: Prj_NVDI
arcpy.DefineProjection_management(NVDI, "GEOGCS['GCS_WGS_1984',DATUM['D_WGS_1984',SPHEROID['WGS_1984',6378137.0,298.257223563]],PRIMEM['Greenwich',0.0],UNIT['Degree',0.0174532925199433]]")

我得到以下错误

Traceback (most recent call last):
  File "C:\egm722\NVDI_arcpy.py", line 59, in <module>
    arcpy.gp.RasterCalculator_sa("Float(\"%b8_clp_prj%\" - \"%b4_clp_prj%\") / (\"%b8_clp_prj%\" + \"%b4_clp_prj%\")", NVDI)
  File "c:\program files (x86)\arcgis\desktop10.5\arcpy\arcpy\geoprocessing\_base.py", line 510, in <lambda>
    return lambda *args: val(*gp_fixargs(args, True))
ExecuteError: ERROR 000539: Error running expression: rcexec() 
Traceback (most recent call last):
  File "<expression>", line 1, in <module>
  File "<string>", line 5, in rcexec
TypeError: unsupported operand type(s) for -: 'str' and 'str'

Failed to execute (RasterCalculator).


Failed to execute (NVDI)

谢谢你的帮助。从我读到的内容来看，这似乎是一个语法错误，但我无法理解。非常感谢你的时间和帮助

Answer 1

arcpy.gp.RasterCalculator_sa("Float(\"%b8_clp_prj%\" - \"%b4_clp_prj%\") / (\"%b8_clp_prj%\" + \"%b4_clp_prj%\")", NVDI)

包括表达式

Float(\"%b8_clp_prj%\" - \"%b4_clp_prj%\")

但两个操作数都是字符串

b8_clp = "C:\\egm722\\b8_clp"
b8_clp_prj = b8_clp
b4_clp = "C:\\egm722\\b4_clp"
b4_clp_prj = b4_clp

在Python中，字符串之间不能相减，因此会出现错误